Multiwalled CNTs

Figure 3.1 (a) Roll-up of a graphene sheet leading to three different types of SWNTs. Reprinted with permission from Ref [25]. Copyright Wiley-VCH Verlag. (b) Different structures of multiwalled CNTs. 

The 0-d nanoparticles can be nano-metal oxides (such as silica,1 titania,2 alumina3), nano-metal carbide,4 and polyhedral oligomeric silsesquioxanes (POSS),5 to name just a few the 1-d nanofibers can be carbon nanofiber,6 and carbon nanotubes (CNT),7 which could be single-wall CNTs (SWCNT) or multiwall CNTs (MWCNT) etc. the 2-d nano-layers include, but are not limited to, layered silicates,8 layered double hydroxides (LDH),9 layered zirconium phosphate,10 and layered titanates,11 etc. 3-d nano-networks are rarely used and thus examples are not provided here. 

For large scale production of carbon nanotubes and nanofibers chemical vapor deposition (CVD) method is most effective. Acetylene, ethylene, propylene, methane, natural gas (consisting predominantly of propane), carbon monoxide were used as a source of carbon [ 1 -8] (in view of large number of publications on CNT synthesis these references are selected arbitrary). Ethylene and possibly propylene are most convenient carbon sources for mass synthesis of high quality multiwall CNT (MWNT). 

Due to their small size and high surface area, nanoparticles can be applied to modify electrode surface property. Convenient and sensitive electrochemical sensors to various targets have been set up by using nanoparticle modification. The determination of acetaminophen in a commercial paracetamol oral solution was reported using a multiwall CNTs composite film-modified glassy carbon electrode with a detection limit of 50 nM (Li etal. 2006a). Heavy metal ions, such as ar-senite (Dai and Compton 2006 Majid et al. 2006) and lead ion (Cui et al. 2005),... 

Figure 6. Electron microscope images. (A) Vertically aligned multiwalled CNT arrays with length about 1 pm. (B) Collapsed CNT arrays after purification process. (C) CNT arrays with SOG after purification and tip opening process. (D) High-resolution transmission electron microscope image of an opened CNT end. From reference 69.

Molecular statics calculations by Buldum and Cfraci [71] support the hypothesis that the observed lock-in orientations are directly related to commensurate registry, and the particular set of commensurate orientations is determined by the CNT chirality (the wrapping orientation of the outer graphene sheet of the CNT). Thus the friction experiments provide a novel method for measuring the nanotube chirality. Large multiwall CNTs of different... 

Figure 19 Computer-generated images of carbon nanostructures showing (A) a spherical C60 fullerene Buckyball structure, (B) a conical form, (C) a SWNT and (D) a cylindrical multiwalled CNT MWNT. Abbreviations SWNT, single-walled nanotube MWNT, multiwalled nanotube. Source From Ref. 102.

Aligned multiwall CNT arrays were synthesized as a basis for a microstructured catalyst, which was then tested in the Fischer-Tropsch reaction in a microchannel reactor [269]. Fabrication of such a structured catalyst first involved MOCVD of a thin but dense A1203 film on a FeCrAlY foam to enhance the adhesion between the catalyst and the metal substrate. Then, multiwall CNTs were deposited uniformly on the substrate by controlled catalytic decomposition of ethene. Coating the outer surfaces of the nanotube bundles with an active catalyst layer results in a unique hierarchical structure with small interstitial spaces between the carbon bundles. The microstructured catalyst was characterized by the excellent thermal conductivity inherent to CNTs, and heat could be efficiently removed from the catalytically active sites during the exothermic Fischer-Tropsch synthesis. 

Carbon nanotubes (CNTs) and carbon nanofibers (CNFs), due to their unique structure and properties, appear to offer quite promising potential for industrial application [236]. As prices decrease, they become increasingly affordable for use in polymer nanocomposites as structural materials in many large scale applications. In fact, three applications of multiwall CNT have been discussed recently first, antistatic or conductive materials [237] second, mechanically reinforced materials [238,239] and third, flame retarded materials [240,241]. The success of CNTs in the field of antistatic or conductive materials is based on the extraordinary electrical properties of CNTs and their special geometry, which enables percolation at very low concentrations of nanotubes in the polymer matrix [242]. 

Pillai et al. described a comparison between two chemical functionalization strategies for the amine functionalization of multiwalled CNTs. The modified CNTs with optimum amine content were used to prepare PLA/CNT nanocomposites through solution casting method. The polymer nanocomposite thus prepared showed improved thermal properties when compared to the neat PLA [65]. 

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