Role of Hydrogen in Catalyst Activation for Plasma-Based Synthesis of Carbon Nanotubes

The importance of hydrogen in carbon nanotube (CNT) synthesis has been known as it supports the critical processes necessary for CNT growth, such as catalyst reduction. However, within the scope of our mini microplasma CNT synthesis reactor, we found that hydrogen was critical for unexpected reasons. Without hydrogen, CNT growth was inhibited and characterized by amorphous carbon particles. Optical emission spectroscopy of the microplasma revealed that without hydrogen, the high-energy electrons induced the immediate decomposition of carbon feedstock simultaneously with the catalyst feedstock, thus suppressing the formation of catalyst nanoparticles and inducing catalyst deactivation. In contrast, the inclusion of hydrogen induced less-immediate decomposition of reactant gases, through the conversion of electron energy of the plasma to thermal energy, which provided the appropriate conditions for catalyst nanoparticle formation and subsequent CNT nucleation. A simple reaction pathway model was proposed to explain these observed results and underlying mechanisms.