Nanomaterials (II): Carbon Nanotubes

Carbon Nanotubes Carbon nanotubes (CNTs) belong to the fullerene family. Fullerenes are composed of covalently boded C atoms arranged to form a closed, convex cage. The first of these molecules C60 was reported in Nature 1985 by Rice University team (Nobel Prize 1996). C60 distinctive soccer ball structure resembled architect Buckminster Fuller s geodesic domes winning the name buckminsterfullerene. CNT is accredited to Sumio Iijima from NEC Corp. in 1991. The basic structure of both single wall CNT (SWNT) and multiwall CNT (MWNT) is derived from planar graphene sheet which is composed of sp2 hybridized C atoms arranged with D6h point group symmetry. SWNT can be imagined to be a sheet that has been wrapped into a seamless cylinder. A typical SWNT diameter is 1.5 nm. It is less common that SWNT diameter is 1 nm or less, and larger tubes are generally more stable than small ones. SWNTs might be hundreds of nm long (aspect ratios is on the order of 1000) and are closed at both ends by hemispherical caps. Half of a C60 molecule is the correct cap for large tubes. MWNTs are essentially multiple SWNTs of different sizes that have formed in a coaxial configuration. MWNTs are typically tens of nanometers in diameter and the spacing between the layered shells in the radial direction of the cyliderical nanotube is approximately ~ 0.3 nm.

(a) The wrapping of a graphene sheet into a seamless SWNT cylinder. (b) and (c) show the aggregation of SWNT in a supramolecular bundles. The cross-sectional view in (c) shows that the bundles have triangular symmetry. (d) A MWNT composed of nested SWNTs. (e) At the macromolecular scale, bundles of SWNTs are entangles.

Selected Characteristics of SWNTs

CNT Synthesis Illustration of the four primary approaches for synthesizing carbon nanotubes: (a) pulsed laser vaporization, (b) arc discharge, (c) catalytic decomposition, and (d) chemical vapor deposition.

Arc Discharge Arc discharge is often used in making MWNTs. A dc voltage is applied between two closely spaced graphite electrodes under an inert atmosphere (e. g., 500 mbar of flowing He). The voltage is sufficiently large to induce dielectric breakdown of the gas molecules between the electrodes, causing a current of ~100 A to flow in the form of an electric arc whose peak temperature is approximately 3000* C. The anode is gradually consumed in this process as C atoms are vaporized from its surface. Ultimately the evaporated atoms redeposit on the cathode, and chamber walls in the form of amorphous C, graphitic nanoparticles, fullerenes, and MWNTs in lesser abundance. The same process can be used to synthesize SWNT if the anode is cored and filled with a mixture of graphite and certain metal catalysts. Transition metals (Co, Ni, Fe) and rare earth elements (Y, Gd) alloyed into the anode at a few atomic percent have successfully catalyzed SWNT growth. Interestingly, SWNTs are essentially never obtained in the absence of catalyst.

Transmission electron micrograph of raw nanotube material produced by arc discharge. The multicomponent reaction product includes (1) isolated SWNTs, (2) SWNT ropes, (3) amorphous or uncatalyzed carbons, (4) residual catalyst particles, and (5) polyaromatic graphitic shells.

Eleanor Campbell, Dept. of Physics, Goteborg University, Carbon Nanotube: Growth, Manipulation and Devices

Arc Production of Carbon Nanoparticles The carbon nanoparticles produced in the flame of a burning candle have a number of potentially useful optical properties, including luminescence in the visible portion of the spectrum, which depends on the particle size. Massimo Bottini, et al., Nature Nanotechnology, 2, 599, Oct., 2007

Pulsed Laser Vaporization (PLV) The apparatus consists of a tube furnace maintained at ~ 1200 C under flowing Ar and high power pulsed laser (typically Nd:YAG operated at 532 nm, 30Hz, and 500 mj per pulse) directed down the furnace tube. A graphite target impregnated with transition metal catalyst is ablated inside the furnace, forming SWNTs that are then collected downstream. The beam can be rastered and the target rotated to maximize utilization of the starting material. First applications of PLV reported 15% conversion of all vaporized C atoms into SWNTs using Co/Ni (0.6/0.6 at. %) and Co/Pt (0.6/0.2 at.%) catalysts. Follow-on work increased the yield 70-90% vol. % by modification of parameters. Unfortunately both arc discharge and PLV methods generate large quantities of unwanted byproduct and require high temperatures (3000-4000 C) to evaporate solid carbon sources, although the nanotubes form at a much lower temperature within the chamber. These results in attempts to circumvent these inefficiencies and, hence, new methods evolved.

Catalytic Decomposition Organometallic precursors are sublimed or evaporated at low temperatures (200-300 C) and delivered into a furnace held at 900-1200 C by an inert carrier gas. As the precursors flow into the heat zone of the reactor they decompose. The metallic part of the organometallic then coarsens into metal clusters or nanoparticles that can catalyze nanotube growth from the organic part. Care must be taken to prevent the metal particles from forming oxide, which, according to some, poison SWNT growth. Ar can be flowed continuously to purge the system of air, and hydrogen (an effective reducing agent) can replace the inert gas to reduce any oxides to elemental metals. Hydrocarbon gases, e. g., methane, hexane, and benzene have been successfully pyrolyzed into SWNTs and MWNTs by metallocene catalysts, and especially by ferrocene. The so-called HiPco (high pressure carbon monoxide) method uses Fe(CO) 5 to catalyze the formation of SWNTs by CO disproprtionation.

Chemical Vapor Deposition (CVD) CVD is essentially similar to catalytic decomposition. The distinction, however, is that CVD often refers to growth on a surface (although this does not necessarily mean that the interface between a growing nanotube and a catalytic substrate is coherent as in epitaxy). CVD processes use a supported catalyst to grow nanotubes along, or off of, a surface. There are several strategies for affixing catalyst particles to substrates. The simplest involves spin-coating a metal salt like Fe(NO 3 ) 3 onto a Si wafer. Alternatively, metal salts can be infused into pore network of alumina or silica. Calcination (heating in air) transforms the metal salts into discrete metal oxide-clusters. Growth is subsequently accomplished in the same manner as in the decomposition method, i. e., in a reaction furnace with flowing hydrogen for catalyst reduction and a hydrocarbon feedstock. The direct evaporation of Fe through shadow mask onto porous Si has also proved effective, enabling the patterned growth of MWNTs.

Atomic force microscopy image of an isolated SWNT deposited onto seven Pt electrodes by spin coating from dichloroethane solution. The substrate is SiO 2. An auxiliary electrode is used for electrostatic gating.

Candace Stuart, Julie Yeiter, Nanotechnology/Carbon Nanotubes, Small Tech 101, 2003, pg.22

Eleanor Campbell, Dept. of Physics, Goteborg University, Carbon Nanotube: Growth, Manipulation and Devices

Structure of CNT CNT is characterized by the chiral vector, C h, C h = na 1 + ma 2 where (n,m) are integers in the zigzag directions a 1 and a 2 q is the Chiral angle with respect to the zigzag axis, i. e., the vector C h and a 1. q determines the degree of twisting of the tube and varies in the range 0 q 30 In terms of integers n and m sinq = 3m 2 n 2 + m 2 + nm tanq = 3m 2n + m cosq = 2n + m 2 n 2 + m 2 + nm

Based on the geometry of the C bonds around the circumference of the tube, there are two limiting cases: Chiral tube corresponding to q = 0 referred to as the armchair. Chiral tube corresponding to q = 0 referred to as the zigzag. For any other q the nanotube is called chiral. In terms of the chiral vector the CNT is zigzag for (n,0) and armchair for (n,n). The CNT diameter, d, may be found as d = 0.783 n 2 + m 2 + nm a = armchair b = zigzag C = chiral

The structure of a SWNT can be visualized as wrapping a one-atom thick layer of graphene into a seamless cylinder around the chiral vector. Depending on n and m the properties of SWNT can vary significantly. The band gap in a SWNT may vary between 0 and 2 ev depending on n and m. Armchair CNTs have electrical properties similar to metals. An armchair CNT is a better conductor than copper. Zigzag and chiral CNTs behave like semiconductors. There are indications from recent research that CNTs may be the best heatconducting material man has ever known. Field emission arises from electron tunneling from a metal tip into vacuum under the application of a strong electric field. The small diameter and high aspect-ratio of the CNT makes it favorable for field emission. Even for moderate voltages, a strong electric field field might develop because of the sharp end of the CNT. The large surface area and high absorbency of CNTs make them ideal for use in sensing and in air, gas, and water filtering.

Filling CNTs A CNT has one characteristic that no other molecule duplicates : it has an interior channel, separated from the exterior environment by an impervious graphene shell. Furthermore, the lumen of most cage molecules like fullerenes is zero-dimensional-the available space is confined in all directions- while the lumen of a CNT is extended in one-dimension. Therefore, if the core of a CNT could be filled with some other atom, ion, or molecule, it would enable the creation of an entirely new class of 1D heterostructure materials. Because so few known materials have such low dimensionality, these entirely synthetic structures could have unexpected properties just as the linear nature of SWNT is responsible for its many enticing qualities. The question is : HOW EXACTLY DO YOU FILL A NANOMETER- SIZED VESSEL?

Filling MWNTs Substantial effort has been devoted to the filling of MWNTs with low surface tension liquids by capillarity. This was first accomplished by a one-step process, in which capped MWNTS were opened and filled with unidentified lead compound by annealing MWNTs decorated with evaporated Pb particles in air at 400 C (above melting temperature for Pb) for 30 mins. Other one-step processes have relied upon wet chemical methods, in which acids and/or chemical precursors of the filling medium were reacted directly with MWNTs for a number of hours. MWNTs have also been filled by direct immersion in a liquid or molten filling medium. Often the encapsulated compound assumes a crystalline structure as seen in the following TEM of a MWNT filled with Sm 2 O 3.

Transmission electron micrograph of a MWNT filled with Sm 2 O 3. The interlayer separation in the MWNT is c.a. 0.34 nm. Lattice planes in the oxide are clearly seen.

Filling SWNTs Mass transport into SWNTs is a fundamentally different challenge due to their smaller diameters. No technique has been developed for the direct synthesis of filled SWNTs The capillarity method used to MWNTs has proved successful at filling SWNTs with Ru, Au, Ag, Pt, Pd, various KCl-UCl4 and AgCl-AgBr compounds, and KI. Unfortunately the surface tension of most liwuids prevents uptake into the subnanometer lumen of a SWNT, limiting the scope of capillarity as a technique for producing filled SWNTs in high yield. However, gases are free of this physical limitation and can easily permeate even nanoscopic pores. This suggests that where possible, a better method for delivering extrinsic species to the inside volume of a SWNT is through the vapor phase.