Cup-stacked nanotubes

Apart from the hitherto described single-waUed and multiwalled carbon nanotubes, there is a variety of further structures with tubular or related geometries. These include bamboo-hke and so-called cup-stacked nanotubes as well as nano-homs and hehcal nanotubes. like normal SWNT and MWNT they consist of curved graphene layers, but they feature additional structural peculiarities that will be considered in the following sections. 

The foregoing case comprises nanotubes closed to the exterior and provided with a pronounced internal structure. The so-called cup-stacked nanotubes, on the other hand, exhibit a very particular surface. They consist of columnar stacks of hollow, truncated cones (Figure 3.29). This structure entails an open external shell... 

Only a few in vivo dermal toxicity studies have been reported so far. Huczko and Lange [50] evaluated the potential of raw CNTs to induce skin irritation by conducting two routine dermatological tests (patch test on 40 volunteers with allergy susceptibilities and Draize rabbit eye test on four albino rabbits). Koyama etal. [51] showed the biological responses to four different types of carbon nanotubes (SWNTs, two types of MWNTs with different diameters, and cup-stacked carbon nanotubes) after their subcutaneous implantation in mice. Both tests [50, 51] showed no or poor irritation effects. However, the in vitro studies in epidermal cell lines exposed to CNTs, and also a more recent report on the toxic outcomes of topical exposure of mice to SWNTs [46], have raised concerns over these assessments. Clearly, this is an area requiring further scientific evaluation. 

Methods of electron spectroscopy are widely used to follow the electron-transfer process. Thus, the progress of electron transfer from naphthalene anion-radical to cup-stacked carbon nanotubes is easily detected by monitoring the UV absorption spectrum. Namely, the absorption band around 500-900 nm due to naphthalene anion-radical completely disappears after reduction of the nanotubes. At the same time, the reduced nanotubes exhibit ESR spectrum characterized with g-factor of 2.0025 (Saito et al. 2006). This g-value is close to the free spin g-factor of 2.0023 that is diagnostic of the delocalized electron on carbon nanomaterials (Stinchcombe et al. 1993). It should be parallelly... 

Waje, M.M. et al.. Durability investigation of cup-stacked carbon nanotubes supported Pt as PEMFC catalyst, FCS Trans., 3, 677, 2006. 

Figure 3.29 Cup-stacked carbon nanotubes consist of individual truncated cones. Model of a cup-stacked CNT (a) and HRTEM-image of these nanotubes (b) ( AlP 2002).

Nevertheless, classical heterogeneous catalysts like particulate noble metals may be immobilized on the nanotube surface as well. Nanoparticles of platinum or rhodium, for instance, can be deposited on cup-stacked carbon nanotubes by reductive precipitation (Figure 3.114b). The catalysts obtained this way suit an application in fuel cells run on methanol. Electrodes made from the nanotube material exhibit twice the efficiency as compared to the classical material XC-72-carbon. The particles of noble metal on the nanotube surface catalyze the direct conversion of methanol into CO2 (MeOH -1- H2O CO2 -1- 6 H -1- 6e ). A material to be employed in such fuel cells has to meet some essential requirements, including a maximal specific surface, a defined porosity and a high degree of crystalhnity. Carbon nanotubes are endowed with exactly these characteristics, which is why they are the most suitable material for electrodes. Their high price, however, is still prohibitive to an industrial scale application. 

Kim C, Kim YJ, Yanagisawa T, Park KC, Endo M (2004) High-performance of cup-stacked-type carbon nanotubes as a Pt-Ru catalyst support for fuel cell applications. J Appl Phys... 

Carbon nanofibers or vapor-grown carbon nanofibers are sp hybridized one-dimensional carbon nanostructures. Three types of carbon nanofiber structures classified based on the angle of graphene sheets are stacked, cup-stacked, and nanotubular [10]. The diameter of carbon nanofibers lies in between carbon nanotubes (100 nm) and carbon fibers (1000 nm). The synthesis procedures used for carbon nanofibers include chemical vapor deposition (CVD). 

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