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. Very similar structures had been observed in 1994 by Peter J.F. Harris, Edman Tsang, John Claridge and Malcolm Green. Ever since the discovery of the fullerene, the family of carbon nanostructures has been steadily expanded. Included in this family are single-walled and multi-walled carbon nanotubes (SWNTs and MWNTs), carbon onions and cones and, most recently, SWNHs. These SWNHs with about 40–50 nm in tubule length and about 2–3 nm in diameter are derived from SWNTs and ended by a five-pentagon conical cap with a cone opening angle of ~20o. Moreover, thousands of SWNHs associate with each other to form the ‘dahlia-like' and ‘bud-like’ structured aggregates which have an average diameter of about 80–100 nm. The former consists of tubules and graphene sheets protruding from its surface like petals of a dahlia, while the latter is composed of tubules developing inside the particle itself. Their unique structures with high surface area and microporosity make SWNHs become a promising material for gas adsorption, biosensing, drug delivery, gas storage and catalyst support for fuel cell. Single-walled carbon nanohorns are an example of the family of carbon nanocones. 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. Very similar structures had been observed in 1994 by Peter J.F. Harris, Edman Tsang, John Claridge and Malcolm Green. Ever since the discovery of the fullerene, the family of carbon nanostructures has been steadily expanded. Included in this family are single-walled and multi-walled carbon nanotubes (SWNTs and MWNTs), carbon onions and cones and, most recently, SWNHs. These SWNHs with about 40–50 nm in tubule length and about 2–3 nm in diameter are derived from SWNTs and ended by a five-pentagon conical cap with a cone opening angle of ~20o. Moreover, thousands of SWNHs associate with each other to form the ‘dahlia-like' and ‘bud-like’ structured aggregates which have an average diameter of about 80–100 nm. The former consists of tubules and graphene sheets protruding from its surface like petals of a dahlia, while the latter is composed of tubules developing inside the particle itself. Their unique structures with high surface area and microporosity make SWNHs become a promising material for gas adsorption, biosensing, drug delivery, gas storage and catalyst support for fuel cell. Single-walled carbon nanohorns are an example of the family of carbon nanocones. SWNHs can be synthesized with high purity by CO2 laser ablation and arc discharge without a metal catalyst. The following two subsections respectively show the representative procedures for the two synthesis methods. The size and purity of the SWNHs can be changed by varying the parameters such as temperature, pressure, voltage and current. CO2 laser ablation technique is utilized to produce the first SWNHs at room temperature in absence of a metal catalyst. 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. Meanwhile, a graphite rod in the middle of the chamber continuously rotates and advances along its axis so that a new surface could be exposed to the laser beam that is vertical to the rod and thus SWNHs are produced. SWNHs can also be prepared by a simple pulsed arc discharge between pure carbon rods in the atmospheric pressure of air and He and Ar with arcing period of 30s. The arc current is set at 120 A and voltage between the electrodes is 15 V. Pre-heating of the carbon rod up to 1000 ℃, is conducted just before ignition of arc to improve the quality of SWNHs. The arc soot deposited on the surface of the chamber is collected and characterized. By this method, the purity of obtained SWNHs is higher than 90%. The mean size of SWNH particles is about 50 nm, which is smaller than those prepared by the CO2 laser method. Soon after the discovery of the SWNHs, the scientists made efforts to study the structure of this new material. In 2000, a detailed X-ray diffraction examination showed that the interhorn-wall distance was 0.4 nm, greater than the interlayer spacing of graphite (0.335 nm). Thus SWNH aggregates should have both microporosity and mesoporosity originating from the above specific structure. An exact surface characterization of SWNHs can extend the application possibilities to secondary energy storage. The pore structure of SWNHs has been extensively studied using simulation and adsorption experiments. The SWNH aggregates have a considerable capacity of micropores and a little mesoporosity due to the hexagonal stacking structure of the SWNHs. 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. The detailed comparison of the simulated adsorption isotherm with the experimental isotherm in the internal nanospaces provided 2.9 nm of the average pore width of the internal nanospaces. 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. In 2002, it was found that nanoscale windows were produced on the wall when the SWNHs were oxidized in oxygen at high temperature,. The size and concentration of these nanoscale windows could be controlled by the oxidation temperature. Besides, the oxidation and compression of SWNHs could induce a pronounced increase in the microporosity and the production of mesopores. Although the intraparticle pore of the original SWNHs is completely closed, 11 and 36% of the intraparticle pore spaces become open by oxidation at 573 and 623 K, respectively. 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. In addition, adsorption analysis can provide a reliable means for evaluation of the pore structure parameters of the interstitial and internal microporosity. The adsorption study showed that the budlike SWNH aggregates possess micropores despite the closed individual nanohorns. A distinctive feature of these micropores is the small average pore width of 1.0 nm. Heat treatment in oxygen opens the closed nanohorns and thus increases the micropore space available for adsorption. 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. Opening mechanism of internal nanoporosity of single-wall carbon nanohorn was revealed through careful oxidation, which allowed controlling the internal nanoporosity. The opening rate was also controllable by oxidation temperature.