B

laser ablation

oven temperature - 1200 °C

(C)

CVD

^nHin

oven temperature 500-1000 "C

Fig. 1 a—c. Schematic experimental setups for nanotube growth methods

Fig. 1 a—c. Schematic experimental setups for nanotube growth methods

The success in producing high quality SWNT materials by laser-ablation and arc-discharge has led to wide availability of samples useful for studying fundamental physics in low dimensional materials and exploring their applications.

1.2 Chemical Vapor Deposition

Chemical vapor deposition (CVD) methods have been successful in making carbon fiber, filament and nanotube materials since more than 10-20 years ago [10,11,12,13,14,15,16,17,18,19].

1.2.1 General Approach and Mechanism

A schematic experimental setup for CVD growth is depicted in Fig. 1c. The growth process involves heating a catalyst material to high temperatures in a tube furnace and flowing a hydrocarbon gas through the tube reactor for a period of time. Materials grown over the catalyst are collected upon

Fig. 2 a,b. Single-walled nanotubes grown by laser ablation (courtesy of R. Smalley)

cooling the system to room temperature. The key parameters in nanotube CVD growth are the hydrocarbons, catalysts and growth temperature. The active catalytic species are typically transition-metal nanoparticles formed on a support material such as alumina. The general nanotube growth mechanism (Fig. 3) in a CVD process involves the dissociation of hydrocarbon molecules catalyzed by the transition metal, and dissolution and saturation of carbon atoms in the metal nanoparticle. The precipitation of carbon from the saturated metal particle leads to the formation of tubular carbon solids in sp2 structure. Tubule formation is favored over other forms of carbon such as graphitic sheets with open edges. This is because a tube contains no dangling bonds and therefore is in a low energy form. For MWNT growth, most of the CVD methods employ ethylene or acetylene as the carbon feedstock and the growth temperature is typically in the range of 550-750°C. Iron, nickel or cobalt nanoparticles are often used as catalyst. The rationale for choosing these metals as catalyst for CVD growth of nanotubes lies in the phase diagrams for the metals and carbon. At high temperatures, carbon has finite solubility in these metals, which leads to the formation of metal-carbon solutions and therefore the aforementioned growth mechanism. Noticeably, iron, r\

Catalyst Support

Fig. 3. Two general growth modes of nanotube in chemical vapor deposition. Left diagram: base growth mode. Right diagram: tip growth mode cobalt and nickel are also the favored catalytic metals used in laser ablation and arc-discharge. This simple fact may hint that the laser, discharge and CVD growth methods may share a common nanotube growth mechanism, although very different approaches are used to provide carbon feedstock.

A major pitfall for CVD grown MWNTs has been the high defect densities in their structures. The defective nature of CVD grown MWNTs remains to be thoroughly understood, but is most likely be due to the relatively low growth temperature, which does not provide sufficient thermal energy to anneal nanotubes into perfectly crystalline structures. Growing perfect MWNTs by CVD remains a challenge to this day.

1.2.2 Single-Walled Nanotube Growth and Optimization

For a long time, arc-discharge and laser-ablation have been the principal methods for obtaining nearly perfect single-walled nanotube materials. There are several issues concerning these approaches. First, both methods rely on evaporating carbon atoms from solid carbon sources at > 3000° C, which is not efficient and limits the scale-up of SWNTs. Secondly, the nanotubes synthesized by the evaporation methods are in tangled forms that are difficult to purify, manipulate and assemble for building addressable nanotube structures.

Recently, growth of single-walled carbon nanotubes with structural perfection was enabled by CVD methods. For an example, we found that by using methane as carbon feedstock, reaction temperatures in the range of 850-1000°C, suitable catalyst materials and flow conditions one can grow high quality SWNT materials by a simple CVD process [20,21,22,23]. High tem perature is necessary to form SWNTs that have small diameters and thus high strain energies, and allow for nearly-defect free crystalline nanotube structures. Among all hydrocarbon molecules, methane is the most stable at high temperatures against self-decomposition. Therefore, catalytic decomposition of methane by the transition-metal catalyst particles can be the dominant process in SWNT growth. The choice of carbon feedstock is thus one of the key elements to the growth of high quality SWNTs containing no defects and amorphous carbon over-coating. Another CVD approach to SWNTs was reported by Smalley and coworkers who used ethylene as carbon feedstock and growth temperature around 800°C [24]. In this case, low partial-pressure ethylene was employed in order to reduce amorphous carbon formation due to self-pyrolysis/dissociation of ethylene at the high growth temperature.

Gaining an understanding of the chemistry involved in the catalyst and nanotube growth process is critical to enable materials scale-up by CVD [22]. The choice of many of the parameters in CVD requires to be rationalized in order to optimize the materials growth. Within the methane CVD approach for SWNT growth, we have found that the chemical and textural properties of the catalyst materials dictate the yield and quality of SWNTs. This understanding has allowed optimization of the catalyst material and thus the synthesis of bulk quantities of high yield and quality SWNTs [22]. We have developed a catalyst consisting of Fe/Mo bimetallic species supported on a sol-gel derived alumina-silica multicomponent material. The catalyst exhibits a surface are of approximately 200 m2/g and mesopore volume of 0.8 mL/g. Shown in Fig. 4 are Transmission Electron Microscopy (TEM) and Scanning Electron Microscopy (SEM) images of SWNTs synthesized with bulk amounts of this catalyst under a typical methane CVD growth conditions for 15 min (methane flow rate = 1000mL/min through a 1 inch quartz tube reactor heated to 900°C). The data illustrates remarkable abundance of individual and bundled SWNTs. Evident from the TEM image is that the nanotubes are free of amorphous carbon coating throughout their lengths. The diameters of the SWNTs are dispersed in the range of 0.7-3 nm with a peak at 1.7 nm. Weight gain studies found that the yield of nanotubes is up to 45 wt.% (1 gram of catalyst yields 0.45 gram of SWNT).

Catalyst optimization is based on the finding that a good catalyst material for SWNT synthesis should exhibit strong metal-support interactions, possess a high surface area and large pore volume. Moreover, these textural characteristics should remain intact at high temperatures without being sintered [22]. Also, it is found that alumina materials are generally far superior catalyst supports than silica. The strong metal-support interactions allow high metal dispersion and thus a high density of catalytic sites. The interactions prevent metal-species from aggregating and forming unwanted large particles that could yield to graphitic particles or defective multi-walled tube structures. High surface area and large pore volume of the catalyst support facilitate high-yield SWNT growth, owing to high densities of catalytic sites

Fig. 4. Bulk SWNT materials grown by chemical vapor deposition of methane. (a) A low magnification TEM image. (b) A high magnification TEM image. (c) An SEM image of the as-grown material

made possible by the former and rapid diffusion and efficient supply of carbon feedstock to the catalytic sites by the latter.

Mass-spectral study of the effluent of the methane CVD system has been carried out in order to investigate the molecular species involved in the nano-tube growth process [25]. Under the typical high temperature CVD growth condition, mass-spectral data (Fig. 1.2.2) reveals that the effluent consists of mostly methane, with small concentrations of H2, C2 and C3 hydrocarbon species also detected. However, measurements made with the methane source at room temperature also reveals similar concentrations of H2 and C2-C3 species as in the effluent of the 900°C CVD system. This suggests that the H2 and C2-C3 species detected in the CVD effluent are due to impurities in the methane source being used. Methane in fact undergoes negligible self-pyrolysis under typical SWNT growth conditions. Otherwise, one would

Fig. 5. Mass spectrum recorded with the effluent of the methane CVD system at 900° C

observe appreciable amounts of H2 and higher hydrocarbons due to methane decomposition and reactions between the decomposed species. This result is consistent with the observation that the SWNTs produced by methane CVD under suitable conditions are free of amorphous carbon over-coating.

The methane CVD approach is promising for enabling scale-up of defect-free nanotube materials to the kilogram or even ton level. A challenge ist wheter it is possible to enable 1 g of catalyst producing 10, 100 g or even more SWNTs. To address this question, one needs to rationally design and create new types of catalyst materials with exceptional catalytic activities, large number of obtain active catalytic sites for nanotube nucleation with a given amount of catalyst, and learn how to grow nanotubes continuously into macroscopic lengths.

A significant progress was made recently by Liu and coworkers in obtaining a highly active catalyst for methane CVD growth of SWNTs [26]. Liu used sol-gel synthesis and supercritical drying to produce a Fe/Mo catalyst supported on alumina aerogel. The catalyst exhibits an ultra-high surface area 540 m2/g) and large mesopore volume 1.4 mL/g), as a result of supercritical drying in preparing the catalyst. Under supercritical conditions, capillary forces that tend to collapse pore structures are absent as liquid and gas phases are indistinguishable under high pressure. Using the aerogel catalyst, Liu and coworkers were able to obtain ~ 200% yield (1 g of catalyst yielding 2 g of SWNTs) of high quality nanotubes by methane CVD. Evidently, this is a substantial improvement over previous results, and is an excellent demonstration that understanding and optimization of the catalyst can lead to scale-up of perfect SWNT materials by CVD.

The growth of bulk amounts of SWNT materials by methane CVD has been pursued by several groups. Rao and coworkers used a catalyst based on mixed oxide spinels to growth SWNTs [27]. The authors found that good quality and yield of nanotubes were obtainable with FeCo alloy nanoparticles. Colomer and coworkers recently reported the growth of bulk quantities of SWNTs by CVD of methane using a cobalt catalyst supported on magnesium oxide [28]. They also found that the produced SWNTs can be separated from the support material by acidic treatment to yield a product with about 7080% of SWNTs.

1.2.3 Growth Mode of Single-Walled Nanotubes in CVD

The states of nanotube ends often contain rich information about nanotube growth mechanisms [22]. High resolution TEM imaging of the SWNTs synthesized by the methane CVD method frequently observed closed tube ends that are free of encapsulated metal particles as shown in Fig. 6. The opposite ends were typically found embedded in the catalyst support particles when imaged along the lengths of the nanotubes. These observations suggest that SWNTs grow in the methane CVD process predominantly via the base-growth process as depicted in figure 3 [10,14,16,22,29]. The first step of the CVD reaction involves the absorption and decomposition of methane molecules on the surface of transition-metal catalytic nanoparticles on the support surface. Subsequently, carbon atoms dissolve and diffuse into the nanoparticle interior to form a metal-carbon solid state solution. Nanotube growth occurs when supersaturation leads to carbon precipitation into a crystalline tubular form. The size of the metal catalyst nanoparticle generally dictates the diameter of the synthesized nanotube. In the base-growth mode, the nanotube lengthens with a closed-end, while the catalyst particle remains on the support surface. Carbon feedstock is thus supplied from the 'base' where the nanotube interfaces with the anchored metal catalyst. Base-growth operates when strong metal-support interactions exist so that the metal species remain pinned on the support surface. In contrast, in the tip-growth mechanism, the nanotube lengthening involves the catalyst particle lifted off from the support and carried along at the tube end. The carried-along particle is responsible for supplying carbon feedstock needed for the tube growth. This mode operates when the metal-support interaction is weak [22].

In the methane CVD method, we have found that enhancing metal-support interactions leads to significant improvement in the performance of the catalyst material in producing high yield SWNTs [22]. This is due to the increased catalytic sites that favor base-mode nanotube growth. On the

Fig. 6 a,b. TEM images of the ends of SWNTs grown by CVD

other hand, catalysts with weak metal-support interactions lead to aggregation of metal species and reduced nanotube yield and purity. Large metal particles due to the aggregation often lead to the growth of multi-layered graphitic particles or defective multi-walled tube structures. Metal-support interactions are highly dependent on the type of support materials and the type of metal precursor being used in preparing the catalyst [22].

1.3 Gas Phase Catalytic Growth

It has also been demonstrated that catalytic growth of SWNTs can be grown by reacting hydrocarbons or carbon monoxide with catalyst particles generated in-situ. Cheng and coworkers reported a method that employs benzene as the carbon feedstock, hydrogen as the carrier gas, and ferrocene as the catalyst precursor for SWNT growth [30]. In this method, ferrocene is vaporized and carried into a reaction tube by benzene and hydrogen gases. The reaction tube is heated at 1100-1200°C. The vaporized ferrocene decomposes in the reactor, which leads to the formation of iron particles that can catalyze the growth of SWNTs. With this approach however, amorphous carbon generation could be a problem, as benzene pyrolysis is expected to be significant at 1200° C. Smalley and coworkers has developed a gas phase catalytic process to grow bulk quantities of SWNTs [31]. The carbon feedstock is carbon monoxide (CO) and the growth temperature is in the range of 800-1200°C. Catalytic particles for SWNT growth are generated in-situ by thermal decomposition of iron pentacarbonyl in a reactor heated to the high growth temperatures. Carbon monoxide provides the carbon feedstock for the growth of nanotubes off the iron catalyst particles. CO is a very stable molecule and does not produce unwanted amorphous carbonaceous material at high temperatures. However, this also indicates that CO is not an efficient carbon source for nanotube growth. To enhance the CO carbon feedstock, Smalley and coworkers have used high pressures of CO (up to 10 atm) to significantly speed up the disproportionation of CO molecules into carbon, and thus enhance the growth of SWNTs. The SWNTs produced this way are as small as 0.7nm in diameter, the same as that of a Ceo molecule. The authors have also found that the yield of SWNTs can be increased by introducing a small concentration of methane into their CO high pressure reactor at 1000-1100°C growth temperatures. Methane provides a more efficient carbon source than CO and does not undergo appreciable pyrolysis under these conditions. The high pressure CO catalytic growth approach is promising for bulk production of single-walled carbon nanotubes.

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