Transformation Of Multiwalled Carbon Nanotubes Research Articles

Multiwalled carbon nanotubes as an additive to Mg-Mg2Si in situ composite obtained by powder sintering

This paper reports on the possibility of applying carbon nanotubes as an additive to metallic materials, where a new phase forms in situ from the powder mixture. Multi-walled carbon nanotubes (MWCNT) amounts of 1 vol% and 3 vol% were applied to synthesize a composite with interpenetrating phases obtained from magnesium microsized powder and nanosized silicon (n)Si by sintering under vacuum. In the experiments, two ordered Mg-(n)Si-MWCNT powder mixtures with a (n)Si: MWCNT ratio of 2: 1 were prepared by sonication. The effects of Mg2Si silicide in situ synthesis on the behavior of MWCNTs and the Mg2Si and Mgα morphology were studied. Using LM with quantitative metallography, SEM with EDS, XRD, HREM, it was found that the MWCNTs were a component of the skeleton formed by submicron and nanosized Mg2Si, MgO, and Mgα mixture. Non-agglomerated MWCNTs were also found in microsized Mg2Si particles in the composite. A decrease in the silicide crystallite size was also observed compared with material without a carbon nanocomponent. The transformation of MWCNTs into another phase or their morphology change was not observed. Therefore, the effects described in this paper can be adapted to technologies focused on different types of composites and for designing Mg2Si-based thermoelectric materials and intermetallic alloys.

Multi-walled carbon nanotube structural instability with/without metal nanoparticles under electron beam irradiation

The structural transformation of multi-walled carbon nanotubes (MWCNT) under electron beam (e-beam) irradiation at room temperature is studied, with respect to a novel passivation effect due to gold nanoparticles (Au NPs). MWCNT structural evolution induced by energetic e-beam irradiation leads to faster shrinkage, as revealed via in situ transmission electron microscopy, while MWCNT surface modification with Au NPs (Au-MWCNT) slows down the shrinkage by impeding the structural evolution process for a prolonged time under the same irradiation conditions. The new relationship between MWCNT and Au-MWCNT shrinking radii and irradiation time illustrates that the MWCNT shrinkage rate is faster than either theoretical predictions or the same process in Au-MWCNTs. As compared with the outer surface energy (positive curvature), the inner surface energy (negative curvature) of the MWCNT contributes more to the athermal evaporation of tube wall atoms, leading to structural instability and shrinkage under e-beam irradiation. Conversely, Au NPs possess only outer surface energy (positive curvature) compared with the MWCNT. Their presence on MWCNT surfaces retards the dynamics of MWCNT structural evolution by slowing down the evaporation process of carbon atoms, thus restricting Au-MWCNT shrinkage. Au NP interaction and growth evolves athermally on MWCNT surfaces, exhibits increase in their size, and indicates the association of this mechanism with the coalescence induced by e-beam activated electronic excitations. Despite their growth, Au NPs show extreme structural stability, and remain crystalline under prolonged irradiation. It is proposed that the surface energy of MWCNTs and Au NPs, together with e-beam activated soft modes or lattice instability effects, predominantly govern all the above varieties of structural evolution.

Electric field induced transformation of carbon nanotube to graphene nanoribbons using Nafion as a solid polymer electrolyte

We report a remarkable transformation of multiwalled carbon nanotubes (MWCNTs, average diameter 40 nm) to graphene nanoribbons (GNRs) in response to a field gradient of ∼25 V/cm, in a sandwich configuration using a solid state proton conducting polymer electrolyte like a thin perfluorosulphonated membrane, Nafion. In response to the application of a constant voltage for a sustained period of about 24 h at both room temperature and elevated temperatures, an interesting transformation of MWCNTs to GNRs has been observed with reasonable yield. GNRs prepared by this way are believed to be better for energy storage applications due to their enhanced surface area with more active smooth edge planes. Moreover, possible morphological changes in CNTs under electric field can impact on the performance and long term stability of devices that use CNTs in their electronic circuitry.

Cutting and Unzipping Multiwalled Carbon Nanotubes into Curved Graphene Nanosheets and Their Enhanced Supercapacitor Performance

We report a remarkable transformation of multiwalled carbon nanotubes (MWCNTs) to curved graphene nanosheets (CGN) by the Hummers method. Through this simple process, MWCNTs can be cut and unzipped in the transverse and longitudinal directions, respectively. The as-obtained CGN possess the unique hybrid structure of 1D nanotube and 2D graphene. Such a particular structure together with the improved effective surface area affords high specific capacitance and good cycling stability during the charge-discharge process when used as supercapacitor electrodes. The electrochemical measurements show that CGN exhibit higher capacitive properties than pristine MWCNTs in three different types of aqueous electrolytes, 1 M KOH, 1 M H(2)SO(4), and 1 M Na(2)SO(4). A specific capacitance of as high as 256 F g(-1) at a current density of 0.3 A g(-1) is achieved over the CGN material. The improved capacitance may be attributed to high accessibility to electrolyte ions, extended defect density, and increased effective surface area. Meanwhile, this high-yield production of graphene from low cost MWCNTs is important for the scalable synthesis and industrial application of graphene. Furthermore, this novel CGN nanostructure could also be promisingly applied in many fields such as nanoelectronics, sensors, nanocomposites, batteries, and gas storage.

Transformation of multi-walled carbon nanotubes to amorphous nano-spheres, micron-size rods and flakes by oxidative sulfation reaction

This study unveils a chemical functionalization provoked phase transformation in the naphthalene modified multi-walled carbon nanotubes (MWCNTs) and the associated formation of amorphous carbonaceous particles of various size and morphology. Functionalized nanotubes were prepared by a sulfonation-oxidization and a highly corrosive oxidative sulfation reaction then characterized by means of thermogravimetry/mass spectrometry, X-ray photoelectron spectroscopy, scanning and transmission electron microscopy and Raman spectroscopy. In spite of the small change in the composition of sulfonation and sulfation reaction mixtures, conventional sulfonation with oleum resulted in lower sulfonation and higher oxidative conversion of MWCNTs accompanied with a mediated exohedral amorphization of the nanotubes, induced by the yield of sp 3 hybridization defect introduction. On the other hand, the newly applied oxidative sulfation reaction provided severe oxidative etching, cutting and shortening of MWCNTs with a subsequent high conversion of sulfate derivatisation of oxidized functionalities, which induced complete collapse and phase transformation of the nanotubes. Amorphization and phase transformation of oxidative sulfated MWCNTs have led to the formation of highly dispersed nano- and micron-size amorphous carbon spheres, rods and flake-like structures. Water based colloidal systems of excellent stability were prepared from the oxidative sulfated carbonaceous material. Consequently, the employed highly corrosive sulfation reaction is capable to provide highly functionalized carbonaceous nano and micro-size particles from MWCNTs.

Structural transformation of multiwalled carbon nanotubes under γ-ray irradiation

Purified multiwalled carbon nanotubes (MWCNTs) were irradiated by 60Co γ-ray with different doses. The structural change of the MWCNTs was revealed by high-resolution transmission electron microscopy and Raman spectroscopy. It was found that under γ-ray irradiation some amorphous structure homogeneously covers the inner tube walls with graphite structure in irradiated MWCNTs. Moreover, the amorphous structure continuously proceeds and the graphite structure is reduced during the γ-ray irradiation till the irradiated MWCNTs become amorphous nanowires with a hollow structure. Based on the interaction between photons and carbon nanotubes, the structural transformation process and the corresponding mechanisms are discussed. In MWCNTs, the collision of a photon with a carbon atom will result in displacement of the atom, i.e. formation of a vacancy (single- or multi- vacancy) and a number of primary knock-on atoms which, if their energy is high, leave the tube or displace other atoms in the MWCNTs. If their energy is low, they can be adsorbed onto the tube walls. These adsorbed atoms play the role of interstitials. All the displaced atoms can be sputtered from the MWSNTs. The carbon atoms sputtered from the MWCNTs can further create some damage in a nearby MWCNTs or be absorbed onto its surface. Along with the simple defects, a number of more complex defects can be formed. The behavior of complex irradiation-induced defects is governed in part by annealing and diffusion of original defects-vacancies and interstitials. The γ-ray irradiation induced structural transformation of MWCNTs was a unique graphite to amorphous structural transition from the outer walls to the inner walls of the irradiated MWCNTs due to the removal of carbon atoms by knock-on displacements.

Graphite-to-amorphous structural transformation of multiwalled carbon nanotubes under proton beam irradiation

The graphite-to-amorphous structural transformation of multiwalled carbon nanotubes (MWCNTs) was investigated under 70 keV proton beam irradiation at room temperature. It was found that under proton irradiation some amorphous structure homogeneously covers the inner tube walls with graphite structure in irradiated MWCNTs. Moreover, the amorphous structure continuously proceeds and the graphite structure is reduced during the proton irradiation until the irradiated MWCNTs become amorphous nanowires with a hollow structure. The proton irradiation induced structural transformation of MWCNTs was a unique graphite-to-amorphous structural transition from the outer walls to the inner walls of the irradiated MWCNTs. The structural evolutionary mechanism of proton-beam-irradiated MWCNTs has been discussed.

Synthesis of diamond nanocrystals by double ions (40Ar+,C2H+6) bombardment

We demonstrate experimentally that an irradiation-induced transformation of multiwalled carbon nanotubes to diamond nanocrystals (DNC) can be realized with double ions (40Ar+，C2H+6) bombardment. This approach is expected to form a new route for synthesis of DNC. The idea of multi-ion irradiation may also be valid for other materials and be used for fabricating nanostructures.

Microstructure Transformation of Multi-walled Carbon Nanotubes during Diamine Grafting Process

Microstructure Transformation of Multi-walled Carbon Nanotubes during Diamine Grafting Process