Experimental characterization of the role of hydrogen in CVD synthesis of MWCNTs

An experimental study was conducted to examine the role of hydrogen in the chemical vapor deposition (CVD) synthesis of multiwalled carbon nanotubes (MWCNTs) in a flow tube reactor using xylene as a carbon source and ferrocene as a catalyst. Ferrocene was introduced into the reactor by two methods. In a single step method, the catalyst was dissolved in xylene and the mixture was introduced using a syringe pump. A two step method was also used where the ferrocene powder was placed in the preheated zone for a certain time to deposit iron catalyst particles on the reactor wall prior to introducing the pure xylene into the reactor. CVD synthesis of carbon products was performed as a function of hydrogen input under constant flow conditions using both methods. SEM and TEM images of the carbon products were examined. The results revealed a competition between the formations of the different carbon products (soot, carbon fibers and CNTs) that altered by the addition of hydrogen. The role of hydrogen is suggested to reduce the rate of carbon production by dehydrogenation so that the more ordered and thermodynamically stable MWCNTs can be produced rather than less ordered and thermodynamically stable soot and carbon fibers.