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synthesis Hydrothermal of multiwall carbon nanotubes Yury Gogotsia) and Joseph A. Libera University of Illinois at Chicago, Department of Mechanical Engineering, 842 West Taylor Street, M/C 251, Chicago, Illinois 60607-7022 Masahiro Yoshimura Tokyo Institute of Technology, Materials and Structures Laboratory, 4259 Nagatsuta, Midori-ku, Yokohama 226, Japan (Received 10 February 2000; accepted 21 September 2000) Multiwall open-end and closed carbon nanotubes with the wall thickness from several to more than 100 carbon layers were produced by a principally new method hydrothermal synthesis using polyethylene/water mixtures in the presence of nickel at 700 800 C under 60 100 MPa pressure. An important feature of hydrothermal nanotubes is a small wall thickness, which is about 10% of the large inner diameter of 20 800 nm. Closed nanotubes were leak-tight by virtue of holding encapsulated water at high vacuum and can be used as test tubes for in situ experiments in transmission electron microscope (TEM). Raman microspectroscopy analysis of single nanotubes shows a well-ordered graphitic structure, in agreement with high-resolution TEM. The hydrothermal synthesis has the potential for producing multiwall nanotubes for a variety of applications. The fabrication of nanotubes under hydrothermal conditions may explain their presence in coals and carbonaceous rocks and suggests that they should be present in natural graphite deposits formed under hydrothermal conditions. Carbon nanotubes1 are among the most exciting new materials being investigated and synthesized because of their potential for use in new technologies and devices.2 In particular, multiwall nanotubes (MWNTs) are of interest because their size can be varied in a wide range, allowing for a variety of applications, including composites, hydrogen storage,3 nano-actuators,4 templates for nanorods/nanowires,5 the next generation of electronic nanodevices,6 and micro-electromechanical systems (MEMS).7 To realize the potential applications of nanotubes, their synthesis techniques must be improved to increase yields and decrease the fabrication cost, as well as to allow a better control over the nanotube diameter and wall thickness. Presently, the carbon arc and chemical vapor deposition (CVD) methods are the most widely used. However, high temperatures, electric fields, evaporation, and vacuum are not necessary conditions to prepare carbon nanotubes. Catalytic CVD synthesis can be conducted at 700 C,8 and electrochemical synthesis of MWNTs was successfully accomplished at 600 C in molten LiCl.9 Hydrothermal synthesis of materials has many advantages over other methods: it is environmentally benign, inexpensive, and allows for reduction of free energies for various equilibria. Supercritical water offers a different chemistry under pressure, sufficient density to dissolve a) Address all correspondence to this author. e-mail: gogotsi@drexel.edu Present Address: Drexel University, Department of Materials Engineering, Philadelphia, Pennsylvania 19104. J. Mater. Res., Vol. 15, No. 12, Dec 2000 materials, a higher diffusivity than in liquid state, a low viscosity facilitating mass transport, and high compressibility allowing for easy changes in density and dissolving power.10 As was found by the authors, carbon coatings can be formed in high-pressure, high-temperature water.11 Taking into account the fact that the formation of hydrothermal carbon can occur under relatively low temperatures (>200 C) and moderate pressures (10 100 MPa),11 this method seems to be very attractive. Some hollow tubelike structures were observed in our previous hydrothermal experiments, but were rare and had a nonuniform cross section.12 Hydrothermal synthesis of bamboolike carbon filaments,13 including well-aligned arrays on a substrate,14 demonstrated the potential of this method for growing carbon nanostructures. The purpose of this work was synthesis of wide-channel nanotubes from a hydrothermal C O H fluid. Various carbon-containing substances as pure chemicals or in mixture with water can be used to achieve the region of the C O H diagram, where solid carbon is in equilibrium with water at the synthesis temperature.12,14 Polymers, such as polyethylene (PE) can be used as a low-cost and convenient carbon sources for fast formation and equilibration of the C H O fluid. PE specimens were placed with deionized water into 3-mm-diameter and 10 50-mm-long Au capsules. The amount of water varied from 0 to 1.6 times the weight of PE. Ni metal powder (3 60%) was added to the capsule since it is known that the iron group metals catalyze growth of nanotubes from the gas phase. The capsule 2000 Materials Research Society 2591 Rapid Communications was heated in a Tuttle-type tube vessel made of a Stellite superalloy at pressure up to 100 MPa of distilled water. In the treatments, the temperature was kept at 700 to 800 C for 2 24 h. Initially, the system was held at a relatively low pressure (60 80 MPa) for a sufficient time to allow for complete pyrolysis of the starting materials, i.e., polyethylene. This was followed by an increase in pressure to 100 MPa, after which time it is believed that the condensation of carbon into nanotubes occurred. The composition and structure of nanotubes were examined using Raman spectroscopy, which is the simplest and most powerful technique for identifying carbon allotropes,15 and electron microscopy. A Renishaw 2000 Raman micro spectrometer with an Ar ion laser (514.5 rim excitation wavelength) was used. The transmission electron microscope (TEM) used was a JEOL 3010 (300 kV) (Tokyo, Japan) with a lattice resolution of 0.14 nm (point resolution 0.17 nm). The microscope is also fitted with a Noran Voyager energydispersive x-ray (EDX) system with a light element x-ray detector, which was used for elemental analysis of nanotubes. MWNTs were dispersed in acetone or toluene and deposited onto a Si wafer or polished alu- minum sample holder for Raman and field emission scanning electron microscope studies, or onto a lacey carbon grid for TEM analysis. MWNTs (Fig. 1) were found in great abundance and with a good reproducibility in our hydrothermal experiments. Although, the yield was not quantified due to small volumes of material produced, 100% of the carbon condensate occurring after the pressure increase was in the form of highly graphitic carbon, which could be flakelike graphite on large particles of Ni or tubular material, which occurred when micro- or nano-sized particles of Ni catalyzed the condensation. Nanotubes prepared in this way may require purification by controlled oxidation16 or wet chemical methods,17 similar to CVD MWNTs. They had typically up to 70 fringes in walls (wall thickness approximately 25 nm), outer diameter from 30 to 200 nm, and inner diameter of up to 160 nm. However, tubes with the diameter of 500 nm and even 1000 nm (wall thickness of about 10% of the diameter and length up to 100 m) were obtained in experiments using minimal amounts of water. These are in the size range of vapor-grown carbon fiber,18 but have a wide channel similar to that of tubular BN.19 They can be labeled as wide-core nano- or microfibers. If no water FIG. 1. TEM micrographs of typical carbon nanotubes produced by hydrothermal treatment of polyethylene at 800 C for 2 h in the presence of 3% Ni powder. The PE/H2O ratio was 1.6. The end of the nanotube shown in (a) comes down to several graphite fringes (b); (d) is the high-magnification lattice fringe image of the tube shown in (c). 2592 J. Mater. Res., Vol. 15, No. 12, Dec 2000 Rapid Communications was added to PE (synthesis from the C H system), thick walls and multiple internal closures of nanotubes were observed. The tubes produced from the C O H system were hollow from tip to tail [Figs. 1(a) and 1(c)], most closed and some open [Fig. 1(a)]. High-resolution images show that some tubes had open ends with tapered wall, with the number of fringes decreasing all the way down to 5 visible fringes [Fig. 1(b)]. In some cases, lattice fringes are inclined relative to the tube longitudinal axis implying chirality [Fig. 1(d)]. Lattice fringe images [Figs. 1(b) and 1(d)] show a high degree of graphitization of hydrothermal nanotubes. Electron diffraction study of these tubes20 showed presence of only finite sets of spots and the absence of diffuse rings that would be expected if amorphous carbon was present. Use of other than PE carbon sources, including solid carbon (e.g., C6021) and ethylene glycol20 can also yield nanotubes. Single-wall nanotubes were not observed in our experiments. Figure 2 shows a Raman spectrum of a single nanotube separated on a Si wafer. The nanometer-diameter MWNTs are observed under optical microscopy as fine black lines, due to diffraction limited optical effect. Micron-sized tubes appear as white lines with dark edges. We found the same spectral signature on all It MWNTs. closely matches spectra from typical microcrystalline graphite (Fig. 2). Relative intensities of Raman bands measured from hydrothermally produced nanotubes are in agreement with those obtained using the arc method,22 showing that they have the degree of perfection similar to that of nanotubes produced at 4000 C. A relatively high full width at half-maximum (FWHM) of the G-band of nanotubes of about 28 cm 1, compared to 18 cm 1 for FIG. 2. Raman spectra of a carbon nanotube, produced by decomposition of PE at 800 C for 2 h in presence of 3% Ni and PE/H2O ratio 1.6, and electrode graphite. The distinctive features of this spectrum include a first order-band at approximately 1580 cm 1 (G-band) with a shoulder at approximately 1620 cm 1. Also prominent are a weak band at about 1350 cm 1 (D-band) and second order features at approximately 2700 cm 1 (2 1350 cm 1) and 3248 cm 1, and a combination mode at 1350 + 1600 2950 cm 1. Comparison of spectra shows that the hydrothermal nanotubes are tubular microcrystals of graphite. microcrystalline graphite and 14 cm 1 for a graphite crystal, may be due to thermal fluctuation line broadening given the high sensitivity of G-band shift with temperature. G-band of nanotubes is typically downshifted compared to the value of 1582 cm 1 expected for graphite due to heating by the laser beam, but at the lowest laser power, nanotubes produce spectra analogous to that of microcrystalline graphite. When measured in air, the G-band shifts as low as 1563 cm 1 above which oxidation occurs and the tubes are cut by the laser. This appeared to be a convenient technique for opening the closed tubes and cutting them to size for applications. Using an inert gas flow to suppress burning of nanotubes, we brought G-band down nearly 40 wave numbers to 1543 cm 1. That shift corresponds to temperature of 1730 C calculated by extrapolating calibrated results on graphite using a hot stage to 700 C. The ability of hydrothermal nanotubes to withstand this temperature demonstrates their high thermal stability. There are several growth mechanisms known, which vary depending on the synthesis technique of the carbon nanotubes.23 C H compounds are completely gasified upon heating in an autoclave.14 In the current experiments, an increase in pressure leads to a decrease of carbon solubility in the hydrothermal fluid10,12 resulting in carbon deposition and growth of nanotubes on catalyst particles. Thus, the growth mechanism is different from the equilibrium ambient-pressure catalytic synthesis, but both have a common feature they require presence of a metal catalyst. Since CH4 and CO are in equilibrium with solid carbon under hydrothermal synthesis conditions,14 growth of nanotubes from these species is assumed. Recent synthesis of nanotubes of equal or larger size in a hot isostatic pressure apparatus at much higher temperatures (approximately 2000 C)24 confirms the influence of pressure on nanotube geometry. However, the pressure-step induced carbon condensation growth mechanism of nanotubes has not been demonstrated before. A very important feature of closed hydrothermal nanotubes is their ability to contain water [Fig. 3(a)] and gases (CO and CH4 are expected from thermodynamic calculations done using ChemSage Gibbs energy minimization code) encapsulated at the synthesis pressure and temperature. They act as miniature pressure vessels and remain gas-tight even under high vacuum in TEM (10 8 torr) showing a very high tensile strength, in agreement with published data,25 and wall perfection required for engineering applications. Heating with the electron beam or a hot stage in TEM enables unique in situ studies of dynamics of condensation, evaporation, phase changes and fluid movement in the tube. If the temperature inside the tube rises above 600 700 C, a chemical reaction between the tube and the hydrothermal fluid starts, which is the reverse process to the one that led to the nanotube 2593 J. Mater. Res., Vol. 15, No. 12, Dec 2000 Rapid Communications FIG. 3. TEM micrographs showing water trapped in closed tubes. (a)The meniscus shows a good wettability of carbon with water (contact angle is <5 ). Fast heating with the electron beam to high temperatures results in a chemical interaction between the tube wall and water-based supercritical fluid leading to (b) dissolution and (c) puncture of the wall. growth. This reaction leads to dissolution of the carbon wall [Fig. 3(b)], the dynamics of which can be monitored in situ until the tube wall puncture [Fig. 3(c)] and loss of the tube content to the microscope environment. EDX shows a decrease of oxygen peak coming from the trapped water to the background level after tube wall puncture. These results demonstrate the successful use of nanotubes for in situ nanofluidic and chemical experiments in TEM. The results of these experiments will be described elsewhere. Recently, nanotubes were found in coal and carbonaceous rock.26 Since the conditions for the formation of coal deposits involve low- or medium-temperature hydrothermal processes at moderate pressures, this work explains why nanotubes can be present in coal. The results of this work also suggest that nanotubes should be present in other hydrothermally formed minerals, including hydrothermal graphite deposits, which are common in nature. Autoclave synthesis of nanotubes under hydrothermal conditions from low-cost raw materials (polyethylene and water) has been demonstrated in this work. Hydrothermal synthesis of carbon nanotubes involves an environmentally benign technology (closed, waterbased systems at moderate temperatures and pressures, low energy consumption and high yield27). A better understanding of the mechanisms of nanotube formation under hydrothermal conditions should make it possible to optimize the conditions for the growth of large quantities of MWNTs, if commercial largevolume autoclaves are used. Large inner diameter of nanotubes and low growth temperature is expected to prevent closure of the tube and allow for growing long nanotubes. ACKNOWLEDGEMENTS sources Center for use of electron microscopes and Raman instrumentation. This work was supported in part by the UIC Campus Research Board and by the Research Institute for Solvothermal Technology, Japan. REFERENCES 1. S. 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