Carbon DWNTs

Finally, Eklund s group has described double-walled nanotubes (DWNT), with ca. 0.7 nm inner diameter and 1.4 nm outer diameter, that can form a three-layer cylindrical capacitor with a shell of bromine anions forming the outer electrode. Holes segregate on the outer nanotube, and saturation bromine doping creates 1 hole per 26 carbon atoms in the DWNT [135]. 

Carbon nanotubes can have one, two, or many sidewalls and are referred to as single-, double-, or multi-walled nanotubes (SWNT, DWNT, or MWNT). Nanotubes can be metallic, or semi-conducting depending on the chirality of the tube. Single-walled nanotubes (SWNT) are about 1 nm in diameter, and hundreds of nanometers long, whereas multi-walled nanotubes (MWNT) are like nested... 

A single-walled carbon nanotube (S WNT) is a single graphene sheet rolled up in a seamless cylinder, whose diameter is of the order of few nanometers (Fig. 4.12). A double-walled carbon nanotube (DWNT) consists of rolled two graphene layers, and a multiwall carbon nanotube (MWNT) exhibits several co-axial rolls of graphene sheets, one sitting in each other and separated by about 0.35 nm. 

SWNTs single-walled nanotubes DWNTs double-waUed nanotubes MWNTs multi-walled nanotubes CNTs carbon nanotubes not purified... 

As is known, surface area of carbon nanotubes, with some assumptions, interrelates with its diameter and number of walls [10]. Taking into account these data, one cat estimate average number of walls to be 2.8 at CNT diameter near 10 nm and surface area 500 m2/g. These parameters are close to reported for doublewall CNT samples containing 50% DWNT produced by Shenzhen NTP (China). 

Double-walled carbon nanotubes (DWNTs), first observed in 1996, constitute a unique family of carbon nanotubes (CNTs). -2 DWNTs occupy a position between the single-walled carbon nanotubes (SWNTs) and the multiwalled carbon nanotubes (MWNTs), as they consist of two concentric cylinders of rolled graphene. DWNTs possess useful electrical and mechanical properties with potential applications. Thus, DWNTs and SWNTs have similar threshold voltages in field electron emission, but the DWNTs exhibit longer lifetimes.3 Unlike SWNTs, which get modified structurally and electronically upon functionalization, chemical functionalization of DWNTs surfaces would lead to novel carbon nanotube materials where the inner tubes are intact. The stability of DWNTs is controlled by the spacing of the inner and outer layers but not by the chirality of the tubes 4 therefore, one obtains a mixture of DWNTs with varying diameters and chirality indices of the inner and outer tubes. DWNTs have been prepared by several techniques, such as arc discharge5 and chemical vapor depo-... 

The smaller diameter carbon nanotubes are known to be less stable than their larger diameter counterparts and tend to oxidize at lower temperatures. Amorphous carbon and carbon nanotubes with defects undergo combustion at lower temperatures, in Figure 4, we show the thermogravimetric analysis (TGA) curves of undoped as well as N- and B-doped DWNTs. The decomposition temperatures of all these doped DWNTs are comparable to but slightly lower that the decomposition temperature of pure DWNTs. Derivative TGA curves also shows the same trend. The slight increase in mass at high temperature may be due to the small metallic impurity. 

Figure 11.1. Crystallinity of PVA-nanotubes composites versus carbon nanotube volume fraction, for various types of nanotubes (DWNT= double-walled nanotubes, OHMWNT= hydroxyled MWNTs, NMWNT= catalytic Nanocyl S.A. MWNTs, OMWNT=catalytic MWNTs from Orleans, AMWNT=Arc grown MWNTs produced in the author s laboratory. Reprinted with permission from reference (23)).

Ceo = Fullerene SWNTs = Single-walled carbon nanotubes MWNTs = Multiwalled carbon nanotubes DWNTs = Double-walled carbon nanotubes CNTs = carbon nanotubes TEM = Transmission electron microscopy HRTEM = High-resolution transmission electron microscopy SEM = Scanning electron microscopy AFM = Atomic force microscopy Ch = Chiral vector CVD = Chemical vapor deposition HiPco process = High-pressure disproportionation of CO RBM = Radical breathing vibration modes DOS = Electronic density of states. 

Figure 6.47. TEM images of MWNTs, formed from the folding (a) 5-, (b) 2- i.e., DWNT), and (c) 7-stacked graphene sheets. The spacing between individual concentric cylinders is 0.34 nm - the distance between adjacent planes in graphite. Reprinted from Dresselhaus, M. S. Etresselhaus, G. Eklund, R C. Science of Fullerenes and Carbon Nanotubes. Copyright 1996, with permission from Elsevier.

Fig. 8 (A) Coexistence of a VGCF and an SWNT (with a diameter of about 20 nm) obtained by the catalytic decomposition of benzene. (From Ref l) The deposition of a partial carbon layer on a carbon nanotube during the thickening process is observed. (B) Double-walled carbon nanotube (obtained by benzene decomposition) and subsequently heat treated at 2800 °C, yielding the same structure as nanotubes prepared by the arc method. (From Ref l) Insert is a schematic diagram of DWNTs. (From Ref (C) Fligh-resolution transmission electron microscope image of two crossing SWNTs coated with amorphous carbons indicates that the structure consists of an individual graphene cylinder in projection. (From Ref. . )...

Recently, DWNTs, which consist of two concentric cylindrical shells, have attracted the attention of numerous scientists because it is believed that these tubes are more thermally and chemically stable and mechanically strong when compared to single-walled carbon nanotubes (SWNTs) (see Figs. 9A and 9C). In addition, as with SWNTs, these double-layered tubes should behave as quantum wires due to its narrow diameters (e.g., <2nm). Through the right combination of judiciously selected catalyst metals using the seeding method (called a catalytic CVD method), which has been considered as a powerful tool... 

MultiwaUed carbon nanotubes (MWNT) consist of a concentric arrangement of single-walled nanotubes with a usually constant distance of layers. There are examples with just two nanotubes fit one into another (sodouble-walled nanotubes, DWNTs) as well as species with many shells (more than 50). The latter measure many nanometers in diameter and may be so hard to distinguish from classical carbon fibers that only electron microscopy can reveal the difference. Common MWNTs, however, possess a smaller number of concentric tubes. The nomenclature of MWNT indicates the intercalation of the inner tubes by the notation (wi,mi) (n2,m2i (n3,m3) ...,starting from the central tube. 

Besides small carbon clusters generated in the reaction zone anyway, the presence of hydrogen further gives rise to light hydrocarbons that contribute to the deposition of DWNT as well. Hence, in principle, this is a floating catalyst CVD performed in situ. It has indeed been applied in a multitude of experiments for the deliberate production of double-walled nanotubes. Normally, acetylene is employed as carbon source because apparently it suits best to surround an existing nanotube with a second layer of amorphous carbon (refer to Section 3.3.6). 

The directed preparation of DWNT on a support of sihcon carbide is feasible by selective evaporation of the sihcon atoms in the uppermost layer of the substrate. The carbon atoms released thereby organize themselves in the shape of double-walled nano tubes that grow up vertically on the surface of the substrate. Here the ahgnment is caused by the close proximity of neighboring tubes that does not allow for a deflection in other directions. 

Figure 3.42 Growth mechanism for double-walled carbon nanotubes (DWNT).

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