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244 MRS BULLETIN/APRIL 2004
Introduction First discovered in 1991,1 carbon nano-
tubes are considered to be a new form of carbon material with many unique electri- cal, mechanical, and chemical properties. They have attracted much attention due to potential applications such as additives for high-strength polymer composites, elec- trode materials for high-capacity batteries, efficient field-emitters as electron sources, and functional components for nanoscale electronic devices. Over the years, basic re- search and applications exploration have been greatly accelerated by the synthesis of high-quality nanotube materials. The first growth of high-quality and milligram- quantity single-walled carbon nanotubes (SWNTs) by laser ablation2 represented an important milestone that enabled the study of the intrinsic properties of nanotubes. It is clear that future developments in nanotube-based science and technology will rely on the highly controlled synthesis of nanotube materials.
Currently, there are four main challenges in the field of nanotube synthesis: (1) the development of low-cost, large-scale proc- esses for the synthesis of high-quality nano- tubes, including SWNTs; (2) control over the structure and electronic properties of the produced nanotubes; (3) control over the location and orientation of the produced nanotubes on a flat substrate; and (4) the development of a thorough understanding of the growth mechanism of nanotubes.
There have been many review articles3–6 and special issues of journals7–9 in addition to several books10–13 focusing on carbon nanotubes published during the past several years. Recently, activities have increased in theoretical modeling and simu- lation of carbon nanotubes in order to un- derstand the growth mechanisms from the theoretical perspective.14–18 In this ar- ticle, we will present an overview of the current state of the art in the synthesis of carbon nanotubes.
The preparation of high-quality SWNTs with high yield has been the goal of many research endeavors. So far, arc-discharge,19,20 laser ablation,2 and chemical vapor depo- sition (CVD) are the three main methods for SWNT production. Arc-discharge and laser ablation were the first methods that al- lowed synthesis of SWNTs in relatively large (gram) amounts. Both methods in- volve the condensation of hot gaseous car- bon atoms generated from the evaporation of solid carbon. However, the equipment requirements and the large amount of en- ergy consumed by these methods make them suitable only on the laboratory scale. The CVD method, which can be easily scaled up to industrial production levels, has become the most important commer- cial method for SWNT production.
Chemical vapor deposition is the term used to describe heterogeneous reactions in which both solid and volatile products are
formed from a volatile precursor through chemical reactions, and the solid products are deposited on a substrate.21–23 It has be- come a common method for thin-film growth on various solid substrates. CVD of carbon has been successful in making car- bon films,24 fibers,25,26 and carbon–carbon composites27 on an industrial scale for more than 20 years; multiwalled carbon nanotube (MWNT) materials28,29 have also been pro- duced using CVD. Only recently has the growth of SWNTs using CVD become possible.30–34
Compared with arc and laser methods, the main advantage of CVD is the more straightforward way to scale up produc- tion to industrial levels. Indeed, Carbon Nanotechnology Inc. (Houston, Texas) is already producing SWNTs on a pound per day scale, using a process called HiPCO (high-pressure catalytic decomposition of carbon monoxide), which is a floating cat- alyst CVD method. Another advantage of CVD methods is that they allow more con- trol over the morphology and structure of the produced nanotubes. With the arc and laser methods, only powdered samples with nanotubes tangled into bundles can be produced. With the CVD methods, we can produce well-separated individual nanotubes either supported on flat sub- strates or suspended across trenches. These nanotubes can be directly used to fabricate nanoscale electronics. For such applications, these well-separated nanotubes present a big advantage over bulk samples, since no separation or purification of the nanotubes is needed. It is known that the purification and separation processes may create de- fects in the nanotubes that can alter their electronic properties. In addition, there are also recent reports that the orientation of the nanotubes35–38 and their diameters39–42 can be controlled by controlling the experi- mental parameters. In the following sec- tions, we will discuss in more detail the various methods for the synthesis of SWNTs. We have divided the methods into two categories: bulk synthesis and sur- face synthesis.
Bulk Synthesis of SWNTs Over the last few years, several methods
have been developed that have the potential for industrial-scale preparation of nano- tubes. All of them are based on CVD meth- ods. Among these methods, four different approaches have been shown to be the most promising: methane CVD, HiPCO, CO CVD, and alcohol CVD.
Methane CVD Methane CVD was developed by Dai’s
group at Stanford; they first reported the synthesis of bulk amounts of SWNTs by
Recent Advances in Methods of Forming Carbon Nanotubes
Jie Liu, Shoushan Fan, and Hongjie Dai
Abstract Since their discovery, carbon nanotubes, both single-walled and multiwalled, have
been a focus in materials research. Fundamental research and application development hinge on high-quality nanotube materials and controlled routes to their organization and assembly.The aim of this article is to provide updated information on recent progress in the synthesis of carbon nanotubes.
Keywords: bulk synthesis, carbon nanotubes, chemical vapor deposition (CVD), multiwalled, single-walled, surface synthesis.
Recent Advances in Methods of Forming Carbon Nanotubes
MRS BULLETIN/APRIL 2004 245
CVD from methane at 900°C.43 A systematic study of the catalyst leads to the conclu- sion that the best catalyst for their process is an Fe/Mo bimetallic catalyst supported on an Al2O3-SiO2 hybrid material. The Al2O3-SiO2 hybrid exhibits both strong metal-support interaction from Al2O3 and better structural stability than either Al2O3 or SiO2 alone at temperatures as high as 900°C. The catalyst has a surface area of al- most 200 m2/g and a mesopore volume of 0.8 mL/g. Weight-gain measurements showed that the yield of SWNTs (the weight of SWNTs divided by the weight of the catalysts) is �35% with a 30 min growth time. Su et al. significantly improved the weight gain of this method to above 100% (30 min growth time) using Al2O3 aerogels impregnated with Fe/Mo nanoparticles as a catalyst.44 The catalyst was prepared by a sol-gel method, followed by supercritical drying. Simply evaporating the solvent of the wet gel would cause a collapse of the pore walls inside the gel by strong forces from surface tension at the liquid/gas inter- face within the pores. The shrinkage would significantly reduce the surface area and pore volume of the dried gel. In the reported method, the solvent in the wet gel is brought to a supercritical state at high temperature and high pressure, where no liquid/gas interface is formed, before being removed from the wet gel. Therefore, the porous structure survives this procedure. The aerogel-based catalyst showed a surface area of 500–600 m2/g. Figure 1a shows a transmission electron microscopy (TEM) image of the SWNTs in the raw product.
Several groups used other hydrocarbons and catalysts to prepare SWNTs. For ex- ample, Hafner et al. prepared SWNTs using an extremely small amount of C2H4 diluted by Ar and an Fe/Mo bimetallic catalyst with an Al2O3 support.33 Both single- and double-walled nanotubes were observed for reaction temperatures from 700°C to 850°C. However, methane is still the most common gas used to prepare SWNTs. It has been shown that when different catalysts are used, the optimal reaction conditions are different. Unfortunately, there is no available systematic theory that can explain all of the experimental observations. These experimental observations tend to be iso- lated from each other, and the observed rules only apply to the specific system. Take methane as an example: when 2.5 wt% Co/MgO catalysts were used, 1000°C is the reaction temperature that produced the best quality nanotubes, as reported by Colomer et al.45 On the other hand, Li et al. prepared high-quality SWNTs by CVD from methane at 850°C on an Fe/Mo catalyst with an MgO support,46 and Harutyunyan et al. reported high-quality
SWNT growth at low temperature (680°C) and low methane flow rate (40 cm3/min).47 Clearly, more systematic studies are needed in this research field to provide a better un- derstanding of the general growth mecha- nism and to explain all of the experimental observations.
HiPCO HiPCO stands for high-pressure catalytic
decomposition of carbon monoxide. It is a method used to prepare SWNTs, using high-pressure CO as the carbon source. The catalysts used in a HiPCO process are formed in the gas phase from a volatile organometallic catalyst precursor intro- duced into the reactor. The organometallic species decompose at high temperature, forming metal clusters on which SWNTs nucleate and grow.
The HiPCO process was originally de- veloped by Smalley’s research group at Rice University.48,49 In this procedure, high-pressure (�30–100 atm) and high- temperature (1050°C) CO with Fe(CO)5 as a catalyst precursor produced high-quality SWNTs at a rate of approximately 450 mg/h. The product consists of entangled SWN