3. Lau, A.K. T., Hui D., The Revolutionary Creation of. New Advanced MaterialsâCarbon Nanotube Compos ites, Composites: Part B, 2002, vol. 33, no. 4, pp.
ISSN 20751133, Inorganic Materials: Applied Research, 2011, Vol. 2, No. 6, pp. 589–595. © Pleiades Publishing, Ltd., 2011. Original Russian Text © A.G. Nasibulin, S.D. Shandakov, M.Y. Timmermans, O.V. Tolochko, E.I. Kauppinen, 2011, published in Voprosy Materialovedeniya, 2010, No. 3, pp. 95–104.
FUNCTIONAL MATERIALS
Synthesis of SingleWalled Carbon Nanotubes by Aerosol Method A. G. Nasibulina, S. D. Shandakovb, M. Y. Timmermansa, O. V. Tolochkoc, and E. I. Kauppinena a
Department of Applied Physics and Center of New Materials, Aalto University School of Science, Finland b Faculty of Physics, Kemerovo State University, Kemerovo, Russia cSt. Petersburg State Polytechnic University, St. Petersburg, Russia
Abstract—Two aerosol synthesis methods of carbon nanotubes are considered. The possibility of separation of individual singlewalled nanotubes from the bundle in the gas phase and control of their parameters with the help of etching agents is shown. The advantage of the aerosol synthesis methods of nanotubes both for variation of parameters of carbon nanotubes (diameter and morphology) and for their subsequent use in high tech areas (electronics, optics, electrochemistry) is discussed. Keywords: carbon nanotubes, methods of synthesis. DOI: 10.1134/S2075113311060104
INTRODUCTION Carbon nanotubes (CNTs) and especially single walled (SWCNTs) have unique mechanical, thermal, and electric properties [1]. SWCNTs are the most durable of all known materials with an exceptionally high modulus of elasticity and tensile strength [2, 3]. For many applications, high heat conductance along the tubes is very important. The possibility of obtain ing both semiconducting and metallic CNTs and their tolerance to the transmission of large electric currents are important properties in micro and nanoelectron ics for creation of fastoperating transistors, memory elements, sensors, and switches and also as conductors in integrated circuits. At present, with growth of C purification NT production, composite materials on their basis are acquiring an ever wider expansion. Depending on the purposes, CNT production may be considered from the point of view of (1) largescale CNT synthesis for their subsequent purification and further use in different applications and (2) synthesis of CNTs of specified purity and morphology for their direct use. Each of these directions has its own pecu liarities and finds its reflection in specified applica tions. The welldeveloped cleaning methods of CNTs promote the advancement of the first approach. In addition, the possibility of CNT functionalization opens new prospects for further manipulation of CNTs in an aqueous or organic medium for their subsequent use or introduction into composite materials. In the recent years, the possibility of CNT separation with respect to their properties into metallic and semicon ducting ones and even extraction of CNTs with speci fied chirality have appeared. However, this approach turns out to be very energy consuming and requires a substantial timetable for purification and preliminary CNT treatment for their preparation for use.
The second approach is aimed towards the synthe sis of CNTs for their direct use, minimizing the inter mediate stages between the synthesis and their use. From the industrial point of view, that implies the syn thesis on a specified carrier or aerosol synthesis with subsequent deposition of CNTs on a substrate on which they will be used. That makes it possible to avoid the laborconsuming stages of CNT cleaning from the catalyst and carrier and also the dispersion of CNTs in the fluids before their deposition on the sub strate. The aerosol method, with the help of which one can obtain relatively clean and highquality SWCNTs in continuous operation, is the most attractive tech nique for the development of this approach. In spite of the large number of publications devoted to CNTs as a whole, the aerosol chemical synthetic methods remain without proper attention in both the foreign and domestic literature. Above all, this is explained by the relative complexity of the experimen tal investigations. The aerosol method requires very exact maintenance of the experimental conditions, since the processes of decomposition of carbon pre cursors and CNT growth in the gas phase are per formed over a few seconds period. The aim of this work is to show the advantage of the aerosol synthesis method of carbon nanotubes both for variation of CNT properties (diameter and morphol ogy) and for their subsequent use in hightech areas (electronics, optics, electrochemistry). EXPERIMENTAL The CNT synthesis was performed with the use of two experimental setups. The first facility (Fig. 1a) is based on the use of decomposition of ferrocene vapor. It is implemented in the form of a vertical reactor in
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which the laminar condition [4] is supported for con trollable growth of catalytic particles and SWCNTs. The ferrocene is evaporated at room temperature by passing CO (with the flow rate of 300 cm3/min) through a cartridge filled with powders of ferrocene and silicon dioxide (inert filler). The flow containing the ferrocene vapor (at pressure of 0.8 Pa) enters a ceramic tube (with inner diameter of 22 mm) through an watercooled socket directly into the hightemperature zone and is mixed with additional CO flow (100 cm3/min). The socket output (24°C) is located in the part of the tube whose walls have a temperature of about 700°C, which provides a large temperature gradient necessary for quick heating of the vapor–gas mixture and growth of small particles of the catalyst. The SWCNTs synthesized with this method are collected either by means of filtra tion of the flow for macroscopic studies or with the use of an electrostatic filter for subsequent studies in a trans mission and a scanning electron microscopes (TEM and SEM, respectively). The second facility is based on use of a hotwire gen erator (HWG). It makes it possible to synthesize both singlewalled CNTs [5] and multiwalled ones [6]. This method is based on the generation of the catalyst particles by means of evaporation of a resistively heated wire (Fe or Ni) and subsequent cooling of the vapor, which leads to nucleation of the supersaturated vapor, its condensation on the clusters, and coagulation. Cat alytic particles grown in such a way are introduced into
the reactor and mixed with the carbon source. This method is presented schematically in Fig. 1b. A ceramic tube with an inner diameter of 22 mm is located within the furnace. For resistive heating, a thin iron wire (0.25 mm in diameter) is used, through which the electric current is passed. The hot wire is located inside of the ceramic tube with an outer and inner diameters of 13 and 9 mm, respectively, and is positioned within a wall temperature of about 400°C. This temperature is optimal for preventing excess growth of particles owing to coagulation and for decreasing the loss of particles on the reactor’s walls. The iron particles forming in the generator’s tube are carried into the reactor in the gas mixture of N2/H2 (or Ar/H2) with mole component ratio of 93 : 7 at a flow rate of 400 cm3/min and are mixed with the external CO flow (400 cm3/min). Inside the reactor besides the reaction of CO disproportionation (1) 2CO CO2 + C(s), ΔH = –169 kJ/mol the reaction occurs also between CO and H2 on the iron particles CO + H2 H2O + C(s), ΔH = –136 kJ/mol. (2) The flow at the reactor output is uniformly diluted with nitrogen (12 l/min) through the porous tube in order to prevent the deposition of the products on the reac tor walls. For synthesis of CNTs, the predetermined temperature is kept in the interval of 570–1500°C with the total gas flow of 800 cm3/min at atmospheric pres
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sure. The mean residence time of the synthesis prod ucts within the reactor is about 2–3 s. Depending on the conditions of the experiment, the SWCNT diame ter is determined by the dimension of the catalyst par ticles and is varied from 1.1 to 2.0 mm [7]. RESULTS AND DISCUSSION The results of TEM observation and optical mea surements showed the formation of a clean SWCNT product in these reactors. As an example, Fig. 2a shows the Raman spectra from the samples synthe sized in the reactor based on decomposition of fer rocene vapor. The presence of the radial breathing mode (RBM) in the lowfrequency region (100– INORGANIC MATERIALS: APPLIED RESEARCH
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200 cm–1) and strong G mode (with a peak at 1592 cm–1) in the Raman spectra indicates the formation of SWCNTs. The noteworthy feature of the spectra is the low intensity of the D mode (about 1300 cm–1), which indicates the small fraction of unordered carbon in the synthetic product. Diameters of nanotubes in the range of 1.2–1.6 nm may be calculated according to the RBM frequency ν as Dn = 238/ν. The first experimental studies of the SWCNT growth by the HWG method with the use of ferrocene vapor showed that the nanotube growth in the reactor truly depends on the state of its walls. It was estab lished that, for providing a stable SWCNT synthesis process, the reactor’s walls should contain the cata No. 6
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lytic material. This may be done either by deposition of the catalyst on the walls or by using a reactor tube produced from the catalytic material (for example, from stainless steel). Further studies showed that for successful SWCNT synthesis, it is necessary to have a small quantity of etching agents, such as CO2 and H2O vapors, which are formed on the reactor’s walls, or they should be introduced from the outside [8–10]. In Figs. 2c and 2d, TEM micrographs are pre sented which show the effect of CO2 additions and water vapor on the length of nanotubes. So, SWCNTs synthesized without etching additions have a length of about 60 nm, whereas in the presence of CO2 and water vapor it exceeds 300 nm. Furthermore, it was revealed that small additions of etching agents lead to considerable changes in CNT diameter. As an example, in Fig. 2b the absorption spectra of SWCNTs synthesized in the reactor based on decom position of ferrocene vapor with different additions of CO2 at the temperature of 890°C are presented. It is apparent that, without addition of CO2, SWCNT growth occurs with a bimodal distribution of diameters with aver age values of 1.1 and 1.2 nm. Upon adding 0.5 and 1.0% CO2, the dimension of the tubes is shifted into the range of larger dimeters—1.4 and 2.0 nm, respectively. Speaking of the morphology, it is important to note that even one individual SWCNT with definite chiral ity located in a predetermined place may be of interest for many applications. From an individual nanotube, one can produce a field effect transistor, memory device, gas sensor, and quantum wire and even use it as a light source. Thus, controllable synthesis methods of individual SWCNTs turn out to be extremely relevant. However, until now the production of devices based on individual nanotubes has been very challenging due to the laborconsuming manipulation process of nano metric objects. In addition, the CNTs upon contact with each other are spontaneously collected into bun dles owing to the Van der Waals interaction, and, as a result, the majority of CNT synthesis methods cannot produce individual tubes. Usually, to separate the bun dles from individual nanotubes, it is necessary to per form additionally the laborconsuming operations of SWCNT ultrasonic treatment, their functionalization, and the use of surfaceactive additions, and only after that can they be deposited on a substrate. However, in ultrasonic treatment, SWCNT shortening occurs, since very high energy is typically used. The function alization of tubes leads to undesirable changes in their electrical and optical properties. The use of surface active additions subsequently demands their careful washout or another removal method. Individual CNTs may be synthesized by the chemical vapor dep osition method directly on substrates. However, the necessity of high temperatures for the synthesis (above 400°C) inevitably limits the choice of substrate mate rials and complicates the integration of CNTs for use in electronic devices.
A onestage process of separation and deposition of individual SWCNTs, synthesized in the gas phase, on to practically any substrate at ambient temperature was shown in [11, 12]. This approach is based on a spontane ous SWCNT charging in the gas phase by means of the bundle formation. It was established that charged SWCNTs at the reactor output under synthesis by the HWG method consist of bundles, and electrically neutral nanotubes are individual ones. The charged SWCNTs may be removed from the gas flow with the help of an electrostatic filter, which consists of two parallel metallic plates with a length of 10 cm, between which at a distance of 1 cm a voltage of about 3–4 kV is applied. The residual fraction of uncharged individual SWCNTs passing through the electrostatic filter are deposited in the corona of the charging device or by means of thermophoresis. Figures 3a and 3b show the TEM microphotographs of SWCNTs collected at the output of the reactor before and after elimination of charged product from the flow. As one can see, the developed method makes it possible to separate individual nanotubes and to collect them on dif ferent substrates, including temperaturesensitive mate rials, for example, polymers. Individual SWCNTs obtained by electrostatic filtering of the bundles were used for fabrication of highly effective field effect transistors. Such transistors, as is known, have very high mobility of charge carriers, but usually show a large hysteresis. This undesirable hysteresis in field effect transistors was used by us for the creation of a memory device [13]. Figures 3c and 3d show a high on/off current ratio (105) of the transistors produced on the basis of indi vidual semiconductoring SWCNTs with the use of a nanometric layer of HfO2 as an insulating layer, grown by atomic layer deposition. Devices designed on the basis of individual SWCNTs make it possible to study the properties of tubes with defined chirality, which is of great scientific interest. But for realization of such devices, it is necessary to use manystage nanolithography processes. For many appli cations, the properties of not one but a group of nano tubes in the form of a thin film may turn out to be more interesting. In this case, the uniformity of SWCNT prop erties will be reached by statistical averaging of single nanotubes (as also nanotube bundless) comprising the film. This gives a reproducible behavior on larger scales as compared with single SWCNTs, the properties of which depend on the chirality. Production methods of different devices and components on the basis of SWCNT films have been successfully demonstrated. So, for example, the group properties of nanotubes have been used for the creation of diodes, elements of logic circuits, solar batter ies, displays, and sensors. Using an aerosol reactor based on decomposition of ferrocene vapor, SWCNTs from the gas phase (with the help of an electrostatic precipitator) were synthe sized and deposited on silicon and polymeric sub strates. After that, thinfilm SWCNT transistors were produced by means of standard lithographic methods,
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which showed fairly good behavior (Figs. 4a, 4b). In spite of the fact that the charge carrier mobility appeared to be less than that in transistors on individ ual nanotubes or on graphene, it by at least one order of magnitude exceeds the mobility in the thinfilm transistors produced with the use of organic semicon ductors. As a whole, this method represents a promis ing trend for development of electronics with the use of flexible and transparent materials [14]. INORGANIC MATERIALS: APPLIED RESEARCH
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Dense films from SWCNTs also have a rather wide spectrum of application. Since the collection of the products in the aerosol method is done at room tem perature, the SWCNTs may be transferred by direct dry printing to practically any material, including polymers, sensitive to increased temperature. For this, it is necessary only to press the SWCNTs between the filter and secondary substrate with a pressure on the order of 103 Pa. In this case, the SWCNT film fully transfers to the substrate. All this is possible owing to No. 6
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poor adhesion of nanotubes to materials which are used for production of filters. For substrates, one can use such flexible materials as polyethylene terephtha late or polyethylene, and also glass, silicon, and vari ous metals. In addition, the use of a mask under the fil ter allows one to obtain a pattern from the SWCNTs with the resolution of up to 200 μm. The maximum dimension of the filter used in our experiment was 28 × 28 cm2. However, if necessary, further enlargement of the filter and substrate dimensions is possible. One of the most interest applications is the use of CNT films of variable thickness as optically transpar ent chemical electrodes. The unique combination of high conductivity, developed surface, low background currents, and electrocatalytic activity in many electro chemical reactions stimulates a wide use of electrodes from SWCNT films in electrochemistry. Singlewalled nanotubes synthesized in the reactor with the use of ferrocene vapor and collected directly from the gas phase on the membrane filter were transferred to a transparent polyethylene substrate by the direct dry printing method based on the simple pressing of the filter to the substrate at room temperature. Obtained in such a way, flexible optically transparent electrodes showed excellent electrochemical properties for a
number of reductionoxidation pairs, overlapping a wide range of potentials (Fig. 4c). Thus, this is a sim ple and easily reproducible method of production of optically transparent electrodes from singlewalled CNTs on flexible and transparent substrates [15]. A similar method of dry transfer of SWCNT film was used for producing a laser modulator with saturation on the basis of SWCNTs. Consistent with our data, this modulator has the widest for today operating carrying capacity of ~1μm. SWCNT film transferred to a silver mirror was used for demonstration of the subpicosec ond operating conditions of a laser with mode locking and use of Yb, Er, and Tm:H for fiber alloying, working at wavelengths of 1.0, 1.56, and 2 μm, respectively. As an example, in Fig. 4d the time dependence of reduced amplitude for a Ybfiber laser after modulation of modes is presented (the pulse width is 0.7 ps) [16]. In [17] a simple method of integration of films from SWCNTs with controllable thickness, transparency, and conductivity into polymer films based on thermal press ing was proposed. In this case, to a polymer heated to temperatures close to melting point, one attaches the film from SWCNTs deposited on the filter, which is eas ily transferred to the surface of the polymer. Obtained in
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such a way, the composite SWCNT/polyethylene mate rial demonstrated good optical transparency and con ductivity and also high mechanical flexibility. In addi tion, this material showed excellent properties of field emission of cold electrons. The relative simplicity of the aerosol synthesis method and the possibility of direct integration of the produced SWCNTs into a final product open up new possibilities for development of this method, and we are sure that, in the near future, this will find direct application in products on the hightech market. CONCLUSIONS
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We have considered briefly two aerosol synthesis methods of CNTs based on the use of the decomposi tion of ferrocene vapor and hotwire generator for syn thesis of iron catalytic particles. The possibility of sep aration of individual SWCNTs from bundles in the gas phase has been shown. In the paper, the possibility of controlling the SWCNT parameters with the help of the etching agents is discussed. One of the main advantages of the aerosol method is the possibility of direct integration of CNT in many applications with out laborintensive cleaning of samples, dispersion in liquids, and their further deposition. Since SWCNTs synthesized by aerosol methods hardly contain amor phous carbon and other undesirable carbonic impuri ties and also have only a small quantity of catalytic metal, which is well “hidden,” i.e., incapsulated inside of nanotubes, they may be used directly in the form in which they leave the reactor. Of most interest for direct SWCNT use are the hightech areas of micro and nanoelectronics for creation of memory elements, thinfilm transistors, chemical electrode sensors, laser modulators, and field emitters of cold electrons.
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ACKNOWLEDGMENTS We are grateful to the staff of Nanomaterials Group: Dr. A. Moysale, A.S. Anisimov, Dr. H. Jian, Dr. J.P. Brown, Dr. D. Gonzalez, Dr. P. Kueypo, Dr. V. Ruiz, A. Kaskela and all those who participated in writing of the original papers on which this review was based. We are also grateful to Dr. K. Grigoras, L.I. Nasi bulin, I. Tian, Dr. M. Rinkio, S. Kivisto, prof. O. Okhot nikov and B. Aytchison. This work was supported by the Federal Agency for Science and Innovation (project no. 02.740.11.5085) and the Academy of Finland (project no. 128 445).
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