Single-walled carbon nanohorn (SWNH or SWCNH) is the name given by Sumio Iijima and colleagues in 1999 to a 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 end with 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 cells. Single-walled carbon nanohorns are an example of the family of carbon nanocones.
Synthesis 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 The CO2 laser ablation technique is utilized to produce the first SWNHs at room temperature in the 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.
Arc discharge
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 °C, 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.
Properties
Porosity
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.
At the same year (2002) as the discovery of nanoscale windows, adsorption isotherms of hydrogen in internal and interstitialspaces of SWNH assemblies were also determined experimentally, which provided the adsorbed density of hydrogen in internal and interstitial spaces. The fact that the adsorbed density of hydrogen in interstitial spaces is lower than that in internal spaces against the prediction from the interaction potential calculation was explained by the self-stabilization effect of the self-locking mechanism.
In 2005, Kaneko et al. declared that the porosity of SWNH assemblies changed after treatment with HNO3. In this case, SWNH assemblies probably possess unavailable interstitial pores at the core of the bundle for adsorption. 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. Moreover, the ultramicroporous SWNH assemblies showed much higher storage capacity of supercritical CH4, showing potential application as a gas storage media.
The detailed structure of SWNHs was further extensively analyzed with X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The considerably strong peak due to single bonding carbons was observed in C1s XPS spectrum of SWNH. This peak intensity increased with oxidation treatment, coinciding with the decrease in Raman G/D intensity ratio. 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. 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.
Electronic properties
The electronic properties are greatly influenced by the unique conical structure of SWNHs. Most of the studies on the electronic properties investigated the conical endcaps that contain five pentagons in the hexagonal network. Berber et al. utilized theoretical calculations to determine the stability, optimum geometry, and electronic properties of SWNHs and found a net electron transfer to the pentagonal sites of the SWNH tips by simulated scanning tunneling microscopy(STM). 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. 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. They investigated the influence of the pentagonal defects on the electronic properties of the SWNHs within the continuum gauge field-theory model. They found that only for five pentagons at the tip does a normalized electron state appear at the Fermi level (a hypothetical level of potential energy for an electron inside a crystalline solid) for an unbounded hyperboloid. The electronic properties of dahlia-SWNHs and oxidized SWNHs were also studied by adsorption of gas such as CO2(an electron donor) and O2 (an electron acceptor). The increased electronic conductivity with adsorption of CO2 indicates that dahlia-SWNHs are n-type semiconductors. On the other hand, the electronic conductivity increases after an initial drop for the oxidized SWNHs, implying that SWNHs can be transformed into p-type semiconductors after oxidation treatment. The initial drop is due to transfer of electrons from CO2 to ox-SWNH annihilates holes, reducing the conductivity, while the later increase is because of the further electron transfer from CO2 after compensation of the hole carriers. As expected, the addition of CO2 results in a decreasing electronic conductivity of the SWNHs.
Magnetic properties The magnetic properties are closely interrelated to the electronic properties in SWNHs. In one electronic spin resonance (ESR) work, two electronic systems were discovered for dahlia-like SWNHs. The first one has a unique temperature-activated paramagnetic susceptibility due to the two-dimensional (2D) graphene-like structure at the surface of the dahlia particles. The second type is due to the disordered graphiticlike interior of the dahlia particles that consist of crushed nanohorns and touching graphene sheets. In this type, the susceptibility is increasing with decreasing temperature until 17 K. This susceptibility is composed of Curie (localized spins) and significant Pauli (conduction electrons, temperature independent) components. Here, the number of localized spins (1.2×10−4 per C atom) is larger than that of multiwall carbon nanotubes (MWNTs) by one magnitude, while the Pauli susceptibility is comparable to that of MWNTs. On the other hand, a large suppression of the paramagnetic susceptibility is observed below 17 K. This phenomenon implies an antiferromagnetic correlation between the localized electrons, in which the localized spins pair into antiferromagnetic singlet pairs. However, the concentration of the localized electrons is too low. To explain this, Garaj et al. suggested that the singlet coupling was mediated by conducting electrons.