Size and temperature effects on the viscosity of water inside carbon nanotubes

The influences of the diameter (size) of single-walled carbon nanotubes (SWCNTs) and the temperature on the viscosity of water confined in SWCNTs are investigated by an "Eyring-MD" (molecular dynamics) method. The results suggest that the relative viscosity of the confined water increases with increasing diameter and temperature, whereas the size-dependent trend of the relative viscosity is almost independent of the temperature. Based on the computational results, a fitting formula is proposed to calculate the size- and temperature- dependent water viscosity, which is useful for the computation on the nanoflow. To demonstrate the rationality of the calculated relative viscosity, the relative amount of the hydrogen bonds of water confined in SWCNTs is also computed. The results of the relative amount of the hydrogen bonds exhibit similar profiles with the curves of the relative viscosity. The present results should be instructive for understanding the coupling effect of the size and the temperature at the nanoscale.     

Polymer blend emulsion stabilization using carbon nanotubes interfacial confinement

The present study demonstrates the effect of unfunctionalized MultiWalled Carbon Nanotubes (MWNTs) interfacial confinement on coalescence suppression in an immiscible polymer blend exhibiting a sea-island morphology. The effect of carbon nanotubes on morphological stabilization in polyamide (PA)/ethylene-methyl acrylate random copolymer (EA) blends is studied using electronic microscopy techniques. Owing to their interfacial localization, MWNTs are shown to enhance both phase dispersion and stability of the dispersed phase for long mixing time (at least 60 min) and very low filler content (0.5 wt.-% MWNTs) compared to what was previously observed in literature. MWNTs also produce a more uniform distribution of droplets size. The main stabilization mechanism proposed is the formation of a deformable barrier network providing a mechanical barrier against coalescence. Blends stabilized by solid anisotropic nanoparticles, like MWNTs, could therefore offer an interesting alternative to blends compatibilized by block-copolymers.     

1D-confinement of polyiodides inside single-wall carbon nanotubes

1D-confinement of polyiodides inside single-wall carbon nanotubes (SWCNT) is investigated. Structural arrangement of iodine species as a function of the SWCNT diameters is studied. Evidence for long range one dimensional ordering of the iodine species is shown by X-ray and electron diffraction experiments independently of the tube diameter. The structure of the confined polyiodides is investigated by X-ray absorption spectroscopy. The confinement influences the local arrangement of the chains. Below a critical diameter Φ_c of 1 nm, long linear polyiodides are evidenced leading to a weaker charge transfer than for nanotube diameter above Φ_c. A shortening of the polyiodides is exhibited with the increase of the nanotube diameter leading to a more efficient charge transfer. This point reflects the 1D-confinement of the polyiodides inside the nanotubes.     

Chain confinement in electrospun nanofibers of PET with carbon nanotubes

Composite nanofibers of poly(ethylene terephthalate), PET, with multiwalled carbon nanotubes (PET/MWCNT) were prepared by the electrospinning method. Confinement, chain conformation, and crystallization of PET electrospun (ES) fibers were analyzed as a function of the weight fraction of MWCNTs. For the first time, we have characterized the rigid amorphous fraction (RAF) in polymer electrospun fibers, with and without MWCNTs. The addition of MWCNTs causes polymer chains in the ES fibers to become more extended, impeding cold crystallization of the fibers, resulting in more confinement of PET chains and an increase in the RAF. The fraction of rigid amorphous chains greatly increased with a small amount of MWCNT loading: with addition of 2% MWCNTs, RAF increased to 0.64, compared to 0.23 in homopolymer PET ES fibers. Spatial constraints also inhibit the folding of polymer chains, resulting in a decrease in crystallinity of PET. For fully amorphous PET/MWCNT composites, MWCNTs do not affect the chain conformation of PET in the ES fibers. For cold crystallized PET/MWCNT composite nanofibers, more trans conformers were formed with the addition of MWCNTs. The increase of RAF (chain confinement) is associated with an increase of the concentration of the trans conformers in the amorphous region as the MWCNT concentration increases in the semicrystalline nanofibers.     

One-dimensional N2 gas inside single-walled carbon nanotubes

The unexpected presence of a linear arrangement of co-axially oriented N₂ molecules inside aligned single-walled carbon nanotubes is revealed by high resolution near-edge X-ray absorption spectroscopy. The encapsulated N₂ molecules exhibit free stretching vibrations with a long electronic lifetime of the X-ray-excited anti-bonding π¹ states. Molecular dynamics simulations confirm that narrow-diameter nanotubes (d < 1 nm) are crucial for stabilizing the linear arrangement of aligned N₂ molecules.     

Scaled-up self-assembly of carbon nanotubes inside long stainless steel tubing

The self-assembly of carbon nanotubes (CNTs) on the inside wall of a relatively long stainless steel tubing for applications such as separations and chromatography, is reported in this paper. The CNTs were deposited by the chemical vapor deposition (CVD) using ethylene as the carbon source and the iron nanostructures in the stainless steel as the catalyst. The coating consisted of a layer of CNTs aligned perpendicular to the circumference of the tubes, often with an overcoat of disordered carbonaceous material, which could be selectively oxidized by exposing the CNT layer below to pure O₂ at 375 °C. Variation in uniformity in terms of the thickness and morphology of the deposited film and surface coverage were studied along the length of a tube by scanning electron microscopy (SEM). The effects of process conditions, such as flow rate and deposition time on the coating thickness, were studied. The catalytic effect of the iron nanostructures depended on surface conditioning of the tubing. It was found that the pretreatment temperature influenced the quality of the nanotube coating. The morphology of the CNT deposit supported the base-growth scheme and VLS (vapor-liquid-solid) growth mechanisms of CNTs.     

Etching effects of ethanol on multi-walled carbon nanotubes

The morphology and microstructure of multi-walled carbon nanotubes (MWCNTs) were modified using ethanol as a mild gas reactant. The etching by OH radicals and deposition of C radicals on the carbon nanotubes were considered to be responsible for the modification of the MWCNT structures and the formation of new carbon nanostructures. The effects of etching and deposition on the MWCNTs were confirmed by using methanol as another gas reactant; this molecule has a higher ratio of hydroxyl radicals to carbon atoms than ethanol. In addition, water vapor, containing no carbon atoms in the molecule, was also applied to etch the MWCNTs as a weak oxidant which resulted in stronger etching effects on the MWCNTs than methanol and ethanol.     

Structural and electronic properties of carbon nanowires made of linear carbon chains enclosed inside zigzag carbon nanotubes

This article presents ab initio self-consistent-field crystal orbital calculations on the structural and electronic properties for recently-discovered carbon nanowires (CNWs) made of linear carbon chains inserted inside zigzag carbon nanotubes using density functional theory. The studies focus on the change of geometric structures and electronic properties upon the encapsulation. It is found that the carbon nanotubes can stabilize the encapsulated carbon chain which prefers a dimerized structure in the tube with larger diameters. The interaction between the tube and the chain becomes more obvious when the tube size decreases, leading to the change of structures and the energy bands upon encapsulation. All the CNWs we calculated are metals with zero band gap. The encapsulation of the carbon chain may modulate the electronic properties for the CNWs depending on the tube size and the filling density of carbon atoms. Therefore, it is expected that CNWs’s electronic properties can be controlled artificially by filling carbon chains with various densities of atoms into the nanotubes.     

Molecular views of physical adsorption inside and outside of single-wall carbon nanotubes

We discuss our own studies of molecular adsorption on and inside of single-wall carbon nanotubes in the broader context of important theoretical and experimental developments in the field. We show that adsorption in the nanotube interior sites as well as in the groove and exterior sites may be resolved by various experimental methods. In addition, the changes that the adsorbate phases undergo due to confinement in the nanotube interior are discussed, particularly focusing on confined molecules of water, alkanes, and an alkene. Attention is also devoted to the use of oxidizing agents such as ozone to open the ends and walls of nanotubes for interior adsorption.     

Ambient effects on the electrical conductivity of carbon nanotubes

We show that the electrical conductivity of single walled carbon nanotubes (SWCNT) networks is affected by oxygen and air humidity under ambient conditions by more than a magnitude. Later, we intentionally modified the electrical conductivity by functionalization with iodine and investigated the changes in the band structure by optical absorption spectroscopy.Measuring in parallel the tubes electrical conductivity and optical absorption spectra, we found that conduction mechanism in SWCNT is comparable to that of intrinsically conducting polymers. We identified, in analogy to conducting polymers, in the infrared spectra a new absorption band which is responsible for the increased conductivity, leading to a closing gap in semiconducting SWCNT.We could show that by different functionalizations of the same SWCNT starting material the properties like conductivity can be dramatically changed, leading to different imaginable applications. We investigated here, an ultraviolet sensor with weakly modified SWCNT.     

Nucleation stability of diamond nanowires inside carbon nanotubes: A thermodynamic approach

Aiming at synthesis diamond nanowires, a simple thermodynamic approach was performed with respect to the effect of nanosize-induced additional pressure on the Gibbs free energy of critical nuclei to elucidate diamond nucleation inside carbon nanotubes upon chemical vapor deposition, based on the carbon thermodynamic equilibrium phase diagram. Notably, these analysis showed that the diamond nucleation would be preferable inside a carbon nanotube due to the effect of surface tension induced by the nanosize curvature of the carbon nanotube and diamond critical nuclei, compared with diamond nucleation on the flat surface of a silicon substrate. Meanwhile, the metastable phase region of diamond nucleation would be driven into a new stable phase region in the carbon thermodynamic equilibrium phase diagram by the effect of nanosize-induced additional pressure. Eventually, we predicted that carbon nanotubes would be an effective path to grow diamond nanowires by chemical vapor deposition.     

Density functional study of the metallization of a linear carbon chain inside single wall carbon nanotubes

By means of the density functional theory, we studied the relaxed structure and electronic properties of new one-dimensional carbon nanostructures conformed by a linear carbon chain (LCC) inside (5,5) and (8,0) single-walled carbon nanotubes (SWCNTs). The calculations were performed with a linear combination of atomic orbitals method using pseudopotentials and the generalized gradient approximation for the exchange-correlation potential. We analyzed the atomic structure, band structure, and the local density of states. We found that, despite the fact that LCC and (8,0) SWCNT have a band gap, the system LCC@(8,0) shows a metallic character. This metallic behavior is provided by the electronic states from the LCC exclusively, due to charge transfer from carbon nanotube to the LCC. However, the electronic characters of the nanotubes in LCC@SWCNT are the same as that of isolated SWCNTs.     

The coupling of metallophthalocyanine with carbon nanotubes to produce a nanomaterial-based catalyst for reaction-controlled interfacial catalysis

A nanomaterial-based metallophthalocyanine catalyst (CoTAPc-MWCNTs) was prepared by covalently immobilizing cobalt tetraaminophthalocyanine (CoTAPc) on multiwalled carbon nanotubes (MWCNTs). The decomposition reaction of H₂O₂ was chosen to investigate the coupled catalytic performance. The results of electron paramagnetic resonance and online electrochemical experiments indicated that the catalytic mechanism of CoTAPc-MWCNTs is different from that of cobalt phthalocyanine molecular catalysts. The catalytic pathway of CoTAPc-MWCNTs involves the following steps: first, electron transfer from CoTAPc to H₂O₂ occurs through the coordination between the central cobalt ion and H₂O₂, reducing H₂O₂ to H₂O; second, electrons are transferred from MWCNTs to the oxidized CoTAPc, forming hole-doped MWCNTs; finally, hole-doped MWCNTs accept electrons from H₂O₂, oxidizing H₂O₂ to O₂. When using N,N-diethyl-1,4-phenylenediamine as a chromogenic substance, one obtained a quantitative estimate of holes injected in MWCNTs, corresponding to 1 hole for about 55 carbon atoms. Furthermore, CoTAPc-MWCNTs exhibit very unusual characteristics of controlled catalysis due to the special hydrophobic surface of the MWCNTs, and can be used as an interfacial catalyst for the determination of H₂O₂ concentration.     

Encapsulating C59N azafullerene derivatives inside single-wall carbon nanotubes

Filling of single-wall carbon nanotubes with C₅₉N azafullerene derivatives is reported from toluene solvent at ambient temperature. The filling is characterized by high-resolution transmission electron microscopy and Raman spectroscopy. The filling efficiency is the same as for C₆₀ fullerenes and the tube-azafullerene interaction is similar to the tube-C₆₀ interaction. Vacuum annealing of the encapsulated azafullerene results in the growth of inner tubes, however no spectroscopic signature of nitrogen built in the inner walls is detected.     

Gold nanoparticles grown inside carbon nanotubes: synthesis and electrical transport measurements

The hybrid structures composed of gold nanoparticles and carbon nanotubes were prepared using porous alumina membranes as templates. Carbon nanotubes were synthesized inside the pores of these templates by the non-catalytic decomposition of acetylene. The inner cavity of the supported tubes was used as nanoreactors to grow gold particles by impregnation with a gold salt, followed by a calcination-reduction process. The samples were characterized by transmission electron microscopy and X-ray energy dispersion spectroscopy techniques. The resulting hybrid products are mainly encapsulated gold nanoparticles with different shapes and dimensions depending on the concentration of the gold precursor and the impregnation procedure. In order to understand the electronic transport mechanisms in these nanostructures, their conductance was measured as a function of temperature. The samples exhibit a ‘non-metallic’ temperature dependence where the dominant electron transport mechanism is 1D hopping. Depending on the impregnation procedure, the inclusion of gold nanoparticles inside the CNTs can introduce significant changes in the structure of the tubes and the mechanisms for electronic transport. The electrical resistance of these hybrid structures was monitored under different gas atmospheres at ambient pressure. Using this hybrid nanostructures, small amounts of acetylene and hydrogen were detected with an increased sensibility compared with pristine carbon nanotubes. Although the sensitivity of these hybrid nanostructures is rather low compared to alternative sensing elements, their response is remarkably fast under changing gas atmospheres.     

In-situ studies of electron field emission of single carbon nanotubes inside the TEM

Electron field emission characteristics of individual multi-walled carbon nanotubes (MWCNTs) were investigated in situ inside the transmission electron microscope (TEM). For a single MWCNT it was found that while field-emission can hardly occur from the side of the nanotube, a curved nanotube may result in finite side emission and the best emission geometry is the top emission geometry. Current-voltage (I-V) measurements made at different vacuum conditions and voltage sweeps emphasize the importance of the adsorbates on the electron field emission of MWCNTs. For a contaminated MWCNT, although the field emission current was reduced, the stability of its emission was improved. A current of up to several tens of μA was observed for a single MWCNT, but it was found that long time emission usually results in drastic structure damage that may lead to sudden emission failure.     

Growth of branch carbon nanotubes on carbon nanotubes as support

A novel method combining the dense fluidized bed and the floating catalytically chemical vapor deposition method (FCCVD) to prepare carbon nanotubes (CNTs) was proposed. Propylene was decomposed at 660 °C, using the CNTs as supports and the metal particles from the in situ pyrolysis of ferrocene as catalyst. The conversion of propylene in this process was closed to 100% under an optimum condition. By SEM and TEM observations, the growth of new generation of CNTs was proven. It was demonstrated that the short and thin CNT branches exist on the tips or sidewalls of CNTs. Based on the analysis of the formation of catalytic sites in the FCCVD in a fluidized bed, a physical model of the formation of branches was proposed.