Differential preheating of hydrocarbon decomposition and water vapor formation shows that single ring aromatic hydrocarbons enhance vertically aligned carbon nanotubes growth

Using a system of multi-zone thermal chemical vapor deposition (CVD) furnaces in parallel and in series, we performed differential preheating of the incoming gases to independently control water vapor formation (from oxygen and hydrogen) and hydrocarbon decomposition to grow vertically aligned multi-wall carbon nanotube (MWCNT) carpets. We identified specific process parameter sweet spots for water vapor formation and for hydrocarbon decomposition that led to over three times taller CNT carpets compared to the reference process. For instance, we performed a kinetic study that showed that additional external preheating of oxygen and hydrogen to form water vapor significantly extended catalyst life, leading to a terminal height approaching 5 mm, while external preheating of the hydrocarbon precursors dramatically hindered CNT growth at high thermal doses. Using gas chromatography – mass spectrometry, we correlated specific carbon species resulting from ethylene decomposition from specific process points to reveal which species favored and which hindered CNT growth. We identified that hydrocarbons composed of one benzene ring connected to different substituents favored CNT growth while hydrocarbons composed of multiple aromatic rings hindered CNT growth. This study can further our understanding of CNT growth mechanisms and help designing complex CVD systems for more efficient synthesis of CNTs.