Directed synthesis of metal catalyzed carbon nanofibers and graphite encapsulated metal nanoparticles is demonstrated where the catalyst metal nanoparticles of Ni have been preformed prior to introduction into a reactive hydrocarbon growth environment. Control of the chemical environment by additives governs the type of product. Use of thiophene as an additive yields carbon nanofibers, whereas chlorobenzene as an additive produces encapsulated metal nanoparticles. Comparison of these results and those without any additive distinguishes the mechanism(s) by which each additive promotes nanotube or encapsulation, respectively.