As-produced SWCNTs

FIGURE 13.5 (a) Schematic of as-produced SWCNT sample showing common impnrities, snch as cata-... 

Surfactants — either anionic surfactants such as sodium dodecylsulfate [SDS], or sodium dodecyl benzene sulfate [SDBS], or polysaccharide [Gum Arabic GA] — were first used to disperse, and exfoliate as-produced SWCNTs in water by ultrasonication, and to stabilize the resulting aqueous CNT suspension, see Figure 2.12. The SWCNTs were synthesized by either the AD method [about 30 % of impurities], or by the HiPCO process [having a catalyst particle content of about 5 wt%]. Please note that not only short surfactant molecules, but also polymeric surfactants such as polystyrene sulfonate, or even conductive polymers having a surfactant nature, can also be successfully used to disperse CNTs in water. 

The laser-ablation method can produce SWCNT under co-evaporation of metals like in the electric arc-discharge method. As metallic catalyst Fe, Co or Ni plays the important role and their combination or addition of the third element such as Y produces SWCNT in an efficient manner. But it is still difficult in the laser-ablation method to produce gram quantity of SWCNT. Nonetheless, remarkable progress in the research of physical properties has been achieved in thus synthesized SWCNT. 

Chapelle et al. 1999(54) Stephan et al. 2000 (55) SWCNT Arc- discharge As-produced Spin casting CNT loading levels 1 to20wt% Raman spectroscopy was used to characterize interaction between SWCNT and PMMA The introduction of PMMA into bundles increases the distance between nanotubes and the interactions between themselves decrease and bundles are destroyed for low concentrations (<2.5 wt%). For higher concentrations (10 wt%) the presence of a high quantity of nanotubes does not allow intercalation by PMMA ... 

Figure 10.2. SEM images of as-produced laser-grown (left) and arc-grown (right) SWCNTs.

CNTs can be made by means of arc-discharge, laser ablation, high pressure carbon monoxide decomposition (HiPCO), and chemical vapor decomposition (CVD) processes. The HiPCO process delivers high quality SWCNTs but the upscaling is difficult. Arc-discharge, laser ablation and CVD systems are currently used to produce SWCNTs as well as MWCNTs in laboratory scales. However, until now only CVD processes have been upscaled successfully to produce MWCNTs in commercially relevant amounts. For this purpose the synthesis is usually performed in fluidized beds [10-12]. Currently (2011), industrial grades of MWCNTs can be purchased for less than 130 USD/kg. 

These two experimental observations point out that the purified CNTs are more bundled at the end of the exfoliation than the as-produced CNTs. This was confirmed by SEM images of the purified HiPCO SWCNTs of the high quality batch, which showed that these... 

Fig. 5. Isolated SWCNT split off from a rope. The diffraction pattern produced by such a single tube is usually too weak to be recorded by present methods. The single graphene sheet in the walls is imaged as a dark line.

SWCNTs have been produced by carbon arc discharge and laser ablation of graphite rods. In each case, a small amount of transition metals is added to the carbon target as a catalyst. Therefore the ferromagnetic catalysts resided in the sample. The residual catalyst particles are responsible for a very broad ESR line near g=2 with a linewidth about 400 G, which obscures the expected conduction electron response from SWCNTs. 

We have reviewed the electronic properties of CNTs probed by magnetic measurements. MW- and SWCNTs can individually be produced, however, the parameters of CNTs are uncontrollable, such as diameter, length, chirality and so on, at the present stage. Since the features of CNTs may depend on the synthesis and purification methods, some different experimental observation on CNT properties has been reported. It is important, however, that most of papers have clarified metallic CNTs are actually present in both MW- and SWCNTs. The characteristic of CESR of SWCNTs is different from that on non-annealed MWCNTs, but rather similar to that on annealed multi-walled ones. The relationship of the electronic properties between SW- and MWCNTs has not yet been fully understood. The accurate control in parameter of CNTs is necessary in order to discuss more details of CNTs in future. 

Optimisation of SWCNT production has been attempted within the framework of the arc-discharge method in which anode and cathode were made of graphite rods, a hole in the anode being filled with metal catalysts such as Y (1 at.%) and Ni (4.2 at.%) [7]. A dense collar deposit (ca. 20% of the total mass of graphite rod) formed around the eathode under He (ca. 500 Torr), with 30 V and 100 A de eurrent. It was eonfirmed that this optimal eollar eontained large amounts of SWCNT bundles eonsisting of (10, 10) SWCNTs (diameter 1.4 nm). Such morphology resembles that produced by the laser ablation teehnique [4,5]. 

Preparation methods for PCNTs have been reviewed in the context of parameters which may lead to large-scale MWCNT synthesis free of by-products. It is noteworthy that the formation of aligned CNTs is currently an active area of research in conjunction with PCNT preparation. The use of SWCNTs and/or MWCNTs in electronic devices are being developed. As yet it has not proved possible to produce CNTs with diameters and helicities to order. The formation of SWCNTs by the PCNT process has not yet been reported and it is of interest to examine whether this process can be used to prepare them. 

As mentioned above, the electronic properties of SWCNTs depend on their chirality and may be semiconducting or metallic. There is still no satisfying way to produce just one sort of SWCNTs, which would require the exact control of catalyst particle size at elevated temperature. Hence, the separation of semiconducting from metallic SWCNTs is of paramount importance for their application in, for example, electric devices, field emission and photovoltaics etc. 

One of the major reasons for the intense interest in CNT is their extreme and varied properties. Table 12.7 contains a comparison between SWCNTs and competitive materials and techniques. Another reason is that CNTs are produced from readily available inexpensive materials and are being considered for use as both bulk materials, such as in composites and clothing, and as components in computers, electrical devices, etc. 

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