Nanotube formation, mechanism

Other evidence of hydrogen formation playing a role includes Fukunaka et al., who were able to observe an increase in Ni nanotube wall thickness when the electrodeposition bath s pH was increased. These nanotubes were produced in an unmodified polycarbonate template. They associate the nanotube formation with the depression of the H2 bubble formation, a schematic of which is found in Figure 10.13. It can be inferred that H2 generation is less at higher pHs resulting in smaller diameter bubble formation and thus thicker-walled nanotubes. Thus, H2 gas bubble formation and/or suppression must be considered as a possible aspect of the metal nanotube formation mechanism. 

In addition, the nanopore filling and nanotube formation mechanism of both polymers and metals is analyzed in detail. The results indicate that many factors affect the deposition morphology including electrochemical bath pH, pore-anchoring agents, nanopore geometry, nanoelectrode morphology, pore-wall functionalization, active ion concentration, as well as electrodeposition potential and/or current density. 

The following mechanism has been proposed for titania nanotube formation by Bavykin and co-workers [94] ... 

Different from the formation mechanism of titania nanotubes, Fe203 nanotubes are formed by a coordination-assisted dissolution process [95]. The presence of phosphate ions is the crucial factor that induces the formation of a tubular structure, which results from the selective adsorption of phosphate ions on the surfaces of hematite particles and their ability to coordinate with ferric ions. 

Yao BD, Chan YF, Zhang XY, Zhang WF, Yang ZY, Wang N (2003) Formation mechanism of TiOz nanotubes. Appl Phys Lett 82 281-283... 

Lange H, Huczko A, Sioda M, Louchev O (2003) Carhon arc plasma as a source of nanotubes Emission specti oscopy and formation mechanism. J Nanosci Nanoteclmol 3 51-62. 

Therefore, multiwalled carbon nanotubes may henceforward be assumed normally to consist of concentric single-walled tubes. These are largely independent of one another and may belong to different structural types even within an individual MWNT. However, this does not mean that alternative structures do not exist. At least partially coiled species have been observed in some cases, and rolled up graphene layers play an important role in some hypotheses on the mechanism of nanotube formation. 

The mechanism of nanotube formation in chemical vapor deposition features characteristics rather distinct from those found for the synthesis by arc discharge or laser ablation. Contrary to the latter, a solution of small carbon clusters in and subsequent diffusion through catalyst particles play a minor role in the deposition from the gas phase. The employed hydrocarbons decompose directly on the surface of the catalytic particle. The carbon, therefore, becomes immediately available for nanotube growth. 

It is conceivable that a parallel bundle of linear polyyne chains (the carbon allotrope named chaoite or carbyne) may form, via thermally-allowed (4 + 2)-n-electron cycloadditions, six-membered rings which lead to graphene sheets and/or nanotubes. However, the mechanism of nanotube formation in a plasma produced by arc discharge appears to require the presence of a transition metal atom at the rim of the growing nanotube [77 j. 

Weng et al. [113] studied the effect of tetramethylammonium cations (TMA"") on HO2 crystal morphology under hydrothermal conditions. The as-synthesized samples were characterized by XRD, TEM and SEM methods (see Table 2). The observed morphologies include besom-like particle, nanosheet and nanotubes. The mechanism to accelerate the formation of nanotube in the base of NaOH/TMAOH mixture is illustrated in Eigure 7. Bulk HO2 is first exfoliated to be layered protonic titanate by the mineralization effect of Na". In the presence of TMA"" cations, the separation of layered protonic titanate is accelerated by intercalating TMA"" cations in layered titanate. As a result of the presence of more layered titanate in the hydrothermal solution, nanotubes are formed ahead of schedule by curliness of layered titanate. Thus, the mechanism through which TMA cations affect crystal growth in the conditions of this study is different. 

It has recently been found that if hydrogen is used as a buffer gas in arc discharge experiments, the formation of C-H bonds at the point of tube growth inhibits nanotube closure [80]. Additionally, the observation that small diameter carbon tubules are contained within larger diameter tubules [81] provides evidence that the inner tube is denied access to a carbon source during growth. These two results support the open-ended mechanism for nanotube formation proposed by lijima [5]. 

Figure 2.18 Typical (a) SEM and (b) TEM of PANI-NTs prepared by dopant-free template-free method. (Reprinted with permission from Macromolecular Chemistry and Physics, Formation mechanism of polyaniline nanotubes by a simplified template-free method by H. Ding,]. Shen, M. Wan andZ. Chen, 209, 8, 864-871. Copyright (2008) Wiley-VCH)...

M. Mazur, M. Tagowska, B. Palys, and K. Jackowska, Template synthesis of polyaniUne and poly(2-methoxyaniUne) nanotubes comparison of the formation mechanisms, Electrochem. Common., 5, 403—407 (2003). 

Y. F. Huang and C. W. Lin, Exploration of the formation mechanisms of polyaniline nanotubes and nanofibers through a template-free method, Synth. Met., 159,1824-1830 (2009). 

Fig. 6.6 SEM images of thermally oxidized NiO with a hoUow DG morphology based on the KirkendaU mechanism, a Cross-section of a NiO nanotube array obteiined by thermal annetding subsequent to template dissolution, b Free-surface of the film shown in (a) with pronounced nanotube formation, c Free-surface of a templated nickel film which was oxidized with the styrenic template present. The template confines the oxidizing Ni struts and thereby, the nanotube void formation...

The last reported diblock copolymer family that formed tubular aggregates in block-selective solvents was poly(phenylquinoline)-fc/ock-polystyrene or PPQ-PS, where PPQ was a rigid-rod block [47]. Such tubes are not discussed further for the following reasons First, the tubes had diameters of several micrometers and were not nanotubes. Second, the formation mechanism and chain packing in such tubes were not well understood at all. While Halperin [48] has developed a scaling theory for micelle formation from rod-coil diblock copolymers with the rod block forming the core, the theory did not apply to the PPQ-PS system as the block-selective solvents used were good for the rod PPQ block rather than the coil PS blocL... 

Z. Wei, Z. Zhang, M. Wan, Formation mechanism of self-assembled polyaniline micro/nanotubes, Langmuir 2002, 18, 917. 

Amelinckx, S., Zhang, X.B., Bemaerts, D., Zhang, X.F., Ivanov, V., and Nagy, J.B. (1994) A formation mechanism for catalytically grown hehx-shaped graphite nanotubes. Science, 265,... 

Fig. 2.4 Formation mechanism for PANI nanotubes and nanofibers synthesized by a self-assembly process (Reprinted with permission from Zhang et al. 2002, Copyright 2002 American Chemical Society)...

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