Formation Mechanism Of Nanotubes Research Articles

Preparation and characterization ofγ-AlOOH nanotubes and nanorods

A solution-phase reaction under hydrothermal conditions atrelatively low temperature is first put forward for the preparation ofγ-AlOOH nanotubes and nanorods, on the basis of the possible strategy that the 2D framework structureof γ-AlOOH containing lamellae could provide orientation for the growth of 1D nanostructures.X-ray diffraction (XRD) patterns, transmission electronic microscope (TEM) images, Fouriertransform infrared (FTIR) spectra, and photoluminescence (PL) spectra were used tocharacterize the products. A possible formation mechanism of nanotubes and nanorodsfrom lamellar precursor is proposed.

Gas‐Phase Production of Single‐Walled Carbon Nanotubes from Carbon Monoxide: A Review of the HiPco Process

AbstractFor Abstract see ChemInform Abstract in Full Text.

Synthesis of Eu2O3 nanotube arrays through a facile sol-gel template approach.

Eu2O3 nanotubes have been successfully fabricated by an improved sol-gel template method within the nanochannels of porous anodic alumina templates. The morphology, structure, and composition of the nanotubes were characterized by means of X-ray diffraction techniques, scanning electron microscope, transmission electron microscopy, and selected-area electron diffraction. The results show that the Eu2O3 nanotubes are polycrystalline with a cubic structure. The outer diameter of nanotubes is 50-80 nm, and the thickness of the tube wall is about 5 nm. The mechanism of nanotube formation was discussed.

Synthesis and Thermal Analyses of TiO2-Derived Nanotubes Prepared by the Hydrothermal Method

TiO2-derived nanotubes were prepared by hydrothermal treatment of TiO2 powder in NaOH aqueous solution. High-temperature x-ray diffraction (HT-XRD) andthermogravimetry-differential thermal analysis (TG-DTA) demonstrated the formation of TiO2 (B) phase (a metastable polymorph of titanium dioxide) from the nanotubes under heating at ∼800 °C, which indicates the as-prepared nanotubes should be composed of layered titanate, most probably as H2Ti3O7·nH2O (n < 3). Dehydration behavior and phase transformation confirmed by the HT-XRD study have suggested reliable reaction path and have well-solved the contradictions on the nanotube-formation mechanism among previous studies.

Gas-phase production of single-walled carbon nanotubes from carbon monoxide: a review of the hipco process.

The latest process for producing large quantities of single-walled carbon nanotubes (SWNTs) to emerge from the Rice University, dubbed HiPco, is living up to its promise. The current production rates approach 450 mg/h (or 10 g/day), and nanotubes typically have no more than 7 mol % of iron impurities. Second-generation HiPco apparatus can run continuously for 7-10 days at a time. In the HiPco process nanotubes grow in high-pressure, high-temperature flowing CO on catalytic clusters of iron. Catalyst is formed in situ by thermal decomposition of iron pentacarbonyl, which is delivered intact within a cold CO flow and then rapidly mixed with hot CO in the reaction zone. Upon heating, the Fe(CO)5 decomposes into atoms that condense into larger clusters. SWNTs nucleate and grow on these particles in the gas phase via CO disproportionation: CO + CO --> CO2 + C (SWNT), catalyzed by the Fe surface. The concentration of CO2 produced in this reaction is equal to that of carbon and can therefore serve as a useful real-time feedback parameter. It was used to study and optimize SWNT production as a function of temperature, pressure, and Fe(CO)5 concentration. The results of the parametric study are in agreement with current understanding of the nanotube formation mechanism.

The effect of hydrothermal conditions on the mesoporous structure of TiO2 nanotubes

A systematic analysis of the influence of preparation conditions in the alkali hydrothermal synthesis on the morphology of TiO2 nanotubes is performed using HRTEM and low temperature nitrogen adsorption. The possible mechanisms of nanotube formation are reviewed and a mechanism based on the key stage of wrapping of intermediate multilayered titanate nanosheets is suggested. The driving force for wrapping is considered to be the mechanical stress arising during crystallisation/dissolution. The average diameter of the nanotubes was found to depend on the temperature and on the ratio of weight of TiO2 to the volume of sodium hydroxide solution. An increase in the temperature from 120 to 150 °C results in an increase in the average nanotube diameter. Subsequent increases in the temperature result in the formation of non-hollow TiO2 nanofibers with an average diameter of 75 nm, a wide distribution in diameter and a length in excess of 10 µm. The increase of the TiO2 : NaOH molar ratio results in an increase in the average diameter of nanotubes and a decrease of surface area. The average inner diameter of TiO2 nanotubes varied between 2 and 10 nm. The pore-size distribution was evaluated from TEM, and low-temperature nitrogen adsorption data using the BJH method. It was shown that nitrogen adsorption is a suitable method for characterisation of the pore morphology of nanotubes.

Fabrication of tapered, conical-shaped titania nanotubes

Using anodic oxidation with a time-dependent linearly varying anodization voltage, we have made films of tapered, conical-shaped titania nanotubes. The tapered, conical-shaped nanotubes were obtained by anodizing titanium foil in a 0.5% hydrofluoric acid electrolyte, with the anodization voltage linearly increased from 10-23 V at rates varying from 2.0-0.43 V/min. The linearly increasing anodization voltage results in a linearly increasing nanotube diameter, with the outcome being an array of conical-shaped nanotubes approximately 500 nm in length. Evidence provided by scanning electron-microscope images of the titanium substrate during the initial stages of the anodization process enabled us to propose a mechanism of nanotube formation.

Dislocation mechanism of nanotube formation

We propose a dislocation mechanism explaining the formation of multi-and single-sheet nanotubes and a disclination mechanism accounting for the formation of nanocones. According to the proposed mechanism, rolling up a graphite sheet gives rise to elastic stresses which are released through the slippage of dislocations with edge and screw components. Each closure of the sheet proceeds by a zipper mechanism and results in the formation of a single-sheet nanotube. The number of dislocation slippages is equal to the number of layers in a multisheet nanotube.

Recent development in electrolytic formation of carbon nanotubes in molten salts

This article reviews the recent research development in the electrolytic production of carbon nano-tubes in molten salts. The experimental procedure and product morphologies of the electrolytic method are described in details. Different hypotheses of the carbon nano-tube formation mechanism in molten salts, particularly it relation with the erosion of the cathode, are compared and discussed. It is anticipated that the electrolytic method can potentially become a cheap and continuous process for the production of curved carbon nano-tubes, carbon sheathed metal nanowires and other carbon based nano-structures.

Synthesis and characterization of self‐doped poly(aniline‐co‐aminonaphthalene sulfonic acid) nanotubes

AbstractSelf‐doped poly(aniline‐co‐aminonaphthalene sulfonic acid) (PANI‐ANSA) was synthesized by the copolymerization of 5‐aminonaphthalene‐2‐sulfonic acid (ANSA) and aniline. Scanning electron microscopy and transmission electron microscopy showed that the morphology of PANI‐ANSA synthesized at a high molar ratio of aniline to ANSA was nanotubular, but at a low molar ratio, only a granular morphology formed. A possible formation mechanism for nanotubes was proposed. PANI‐ANSA had better thermal stability than HCl‐doped polyaniline; the highest onset decomposition temperature was as high as 340°C because of SO3H linked with the polymer backbone by a covalent bond. PANI‐ANSA was partially soluble in basic solutions, and its conductivity was between 10−2 and 10−4 S/cm, depending on the sulfonation degree. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 1297–1301, 2003

Microstructure and formation mechanism of titanium dioxide nanotubes

Titanium dioxide nanotubes are synthesized using sol–gel method. Electron energy-loss spectroscopy has been used to investigate the chemical composition of the nanotubes. The results show that the atomic ratio of O/Ti is very close to two. Electron diffraction studies indicate that the nanotubes have the anatase structure. High-resolution transmission electron microscopy studies show that the nanotubes possess a layered structure with layer spacing of about 7.1 Å. The tube axis is determined to be along [0 1 0] direction of the anatase phase. The formation mechanism of the nanotubes can be explained as 3D→2D→1D. The two-dimensional lamellar TiO 2 is essential for the formation of the nanotubes.

Rate-Limiting Processes in the Formation of Single-Wall Carbon Nanotubes: Pointing the Way to the Nanotube Formation Mechanism

Four rate-limiting processes for the formation of single-wall carbon nanotubes (SWCNT) could be identified by varying furnace temperature, gas type, and pressure in a pulsed-laser evaporation setup. One rate-limiting process accounts essentially for all relevant gas-pressure dependencies and can be quantitatively described using a single gas-specific constant. One thermally activated process is related to fullerene formation, whereas another process, following a T2-law, is discussed in terms of the diffusion of carbon through molten catalyst nanoparticles. The data provide strong support for an “undercooled melt” mechanism of nanotube formation.

Single-walled carbon nanotubes formation with a continuous CO 2 -laser: experiments and theory

It has been proved [1] that the use of a CO2-laser system operating in continuous wave mode (cw) can be efficiently used for the production of carbon single-walled nanotubes (SWNTs). In this article we first describe in detail the variable experimental conditions (different ambient gases, static gas pressure, and gas flow) for SWNT formation and summarize the results of the characterization studies of the synthesized materials. Second, we analyze the influence of the different experimental conditions on the SWNTs formation process. We show that the heat transport, kinetic, and diffusion processes allow us to explain seemingly different formation conditions in a qualitative and semi-quantitative agreement with the experimental results. The presented self-consistent scenario for nanotube formation in a gas phase allowed us to propose future experiments on testing the mechanism of nanotube formation.

Fullerene blacks and cathode deposits derived from plasma arcing of graphite with naphthalene

Fullerene black and cathode deposits formed by arcing graphite with naphthalene have been examined. Micro-Raman, transmission electron microscopy and differential thermal analysis show that fullerene blacks produced in the presence of naphthalene are more graphite like than those prepared in its absence. These techniques also show that the cathode deposits are altered by the presence of naphthalene in the feed material. The mixture of nanotubes so formed has a larger average void size than that formed in the absence of naphthalene. The results support proposed mechanisms of nanotube formation that involve growth by incorporation of carbon atoms into open tubes. They also show that naphthalene can be directly incorporated into fullerene black and thereby increase the number of hexagonal sheet structures in the carbon deposit.

Formation Mechanism of Nanotubes in GaN

A formation mechanism for so-called nanotube defects in GaN is proposed. It is shown that two related types of defects are formed: nanotubes and pinholes. Both start with $\mathsf{V}$ shaped facets on ${10\overline{1}1}$ polar planes. Slow growth rate on these polar planes and impurity poisoning of growth steps are suggested as being responsible for initiation of these defects.