Laser-induced Reactions in Acetylene Flow: Shell- Shaped Carbon Nanoparticle Formation

Recently it was reported that infrared CO2 CW laser irradiation of acetylene flow can produce continuous layer shell-shaped carbon nanoparticles (SCNP) directly from the gas when the laser power exceeds some critical value [Choi et al., 2004]. The transition process is distinguished by the blazing light emission in visible, indicating high electronic excitations in reacting molecules. It is remarkable that acetylene does not absorb the 10.6 μm wavelength laser radiation and a self-sustained reaction should be initiated firstly. After the start-up the process goes though laser-absorbing intermediates leading to SCNP production. It was found in extra situ experiments that the SCNPs show the famous ultraviolet (UV) absorption peak at 220 nm, known from interstellar media absorption, as the consequence of the existence of pentagonal topological defects in the continuously closed graphene layers [Pikhitsa et al., 2005]. This perfect continuity is the main feature that distinguishes SCNPs from soot. Yet, the mechanism of the SCNP generation has not been explained. In particular, the question why, instead of chaotic stacks of basic structural units (BSU) characteristic for soot, the perfect continuous carbon layers are formed, remained unanswered. Although it is known that acetylene (after a start-up heating) can be exothermically decomposed into acetylene carbon black and hydrogen, it is unlikely that this thermal reaction alone could self-sustain, considering short residence time of less than 1 msec in the laser beam and temperatures as low as 500 C at 1 mm from the reaction zone so that even the residual hydrocarbons do not burn when contacting air downstream and can be observed in UV and infrared (FTIR). Also acetylene black particles produced in standard way by surface reactions may look similar to SCNPs from [Choi et al., 2004] only after a prolonged high temperature post treatment. Our further experimenting with acetylene flow shielded from air with inert gases revealed the potentials of the process – it occurs that there are conditions when a variety of carbon allotropes can be generated: giant fullerenes of 5 nm (Fig. 1), cubic nanodiamonds (Fig. 2), carbon nanotubes (Fig. 3) and amorphous carbon seen in Fig. 3 and also in Fig. 4 together with giant fullerenes. As fas as nanodiamonds (Fig. 2) are present in our experiment one may conclude that it can not be laser heating that is responsible for the generation of these nanostructures but instead it is multiphoton infrared laser photochemistry (MP) that is involved in production of amorphous carbon and nanodiamonds.