Single-walled carbon nanohorn
Single-walled carbon nanohorn (SWNH or SWCNH) is the name given by Sumio Iijima and colleagues in 1999 to horn-shaped sheath aggregate of graphene sheets.
The CO2 laser ablation generator is composed of a high-power CO2 laser source (with a wavelength of 10.6 μm, 5 kW of power, 10 nm of beam diameter, and the pulse width varies from 10 ms to continuous illumination) and a plastic-resin reaction chamber attached with a vacuum pumping system, inlet and outlet gas valves and a ZnSe lens system to adjust beam intensity.
Ar gas is introduced and flowed through the inside chamber to remove the products to the collection filter under the pressure of 760 Torr at room temperature.
[15] In 2001, N2 adsorption was observed in the internal nanospace and on the external surface of the single SWNH particle, studied by grand canonical Monte Carlo simulation and was compared with the experimental results.
[16] The high-resolution N2 adsorption analysis could clearly elucidate the presence of internal nanopores, external micropores of the triangular arrangement of three particles, and interparticle mesopores in the assembly structure for partially oxidized SWNHs.
As the number and size of the windows in the wall of SWNH can be varied by the heating temperature, the possibility for a molecular selective adsorbent is shown.
[21] In addition, adsorption analysis can provide a reliable means for evaluation of the pore structure parameters of the interstitial and internal microporosity.
The oxidation affects mostly the closed pores by creating windows on the walls and does not change the bundle structure as well as the interstitial microporosity.
The intercalation of HNO3 into such narrow interstitial spaces resulted in an increase of the pore volume, which developed the microporosity, thus highly ultramicroporous SWNH assemblies were successfully prepared.
It was concluded that the presence of a considerable amount of single bonding carbons was the reason for the unique assembly structure accompanying with a strong D-band in Raman spectrum of SWNHs.
[26] The inside structure of SWNHs was examined by electron microscopy observations after focused ion beam (FIB) cutting.
It was revealed that the interior consists of disordered single-layered graphene sheets with a lateral size of up to 10 nm and an interlayer distance of approximately 4–5 Å.
The local density of electronic states at the tip varies corresponding to the shapes of the SWNHs that differ in the relative locations of the five pentagons.
[29] Pursuing this further, Kolesnikov et al. proposed a hyperboloid geometry that has a cone asymptotic at large distance and a smoothing at the tip for SWNHs.
The second type is due to the disordered graphiticlike interior of the dahlia particles that consist of crushed nanohorns and touching graphene sheets.
Normally, a large diamagnetism is expected for the sp2 bonded carbon materials due to the existence of π-electron orbital magnetism.
[32] Various methods have been developed to functionalize carbon nanohorns including covalent bonding, π-π stacking, supramolecular assembly and decoration of metal nanoparticles.
Similarly, organic π-electron donor, tetrathiafulvalene (TTF-) could be assembled onto SWNHs through coulombic attraction to form a water-soluble nanohybrid with positively charged pyrene (pyr+) as a medium.
These oxygenated groups could react with the protein bovine serum albumin to form bio-conjugates which were highly dispersed in phosphate-buffered saline and could be taken up by cultured mammalian cells via an endocytosis pathway.
The electronic properties of the porphyrin/nanohorn assemblies (SWNH/H2P) have been investigated by a combination of several techniques to show the electron-transfer process between the porphyrins and the carbon nanostructures.
Functionalized carbon nanohorns show better dispersity and when bio-conjugated, they can serve biomedical applications such as probing, imaging and drug delivering.
[45] Another hydrogen peroxide biosensor was fabricated using soybean peroxidase decorated SWNHs modified electrode based on the realization of direct electrochemistry of enzyme.
For example, location of maximum axial normal stress becomes closer to the tip of CNH as the cone angle increases (Figure on the right).
Attributed to its distinctive dahlia-flowerlike structure and already desirable size (usually <100 nm), SWNHs are a potential vehicle for intracellular delivery.
[58] Polyethylene glycolcould bind to the hydrophobic surface of SWNHs to enhance their dispersibility in water for further application in drug delivery.
Pt nanoparticles with diameters less than 5 nm could be well dispersed on SWNHs and this catalytic nanohybrid was useful for the power generation by polymer electrolyte fuel cell.
Employing Pt nanoparticles supported on functionalized TiO2 colloidal spheres with nanoporous surface as cathode catalyst, the as-assembled glucose/O2 biofuel cell operate at the physiological condition with good performance.
[63] Hydrogen storage is a key enabling technology for the advancement of fuel cell power systems in transportation applications.
The electrode configuration and duration of arc discharge were modified in order to enhance the yield and methane-adsorption properties of SWNHs.
Since their long-term stability is comparable to that of nanotubes, nanohorns could represent an enticing alternative for field emission applications that do not require high current densities.
