This work deals with the catalytic decomposition of carbon-containing gases (ethylene) over nickel catalysts to grow graphitic capsules at relatively low temperatures (853 K). The influence of different experimental conditions, such as catalyst size, reactant flow ratio (C2H4:O-2), gas flow rate and temperature, is studied. The size of the nickel particles determines the type of structures; while nano-sized nickel particles favor the growth of nanofilaments, graphitic capsules occur over nickel microparticles. The use of reducing and oxidizing environments in the growth of graphitic structures over microparticles are compared, the latter being more appropriate for growing nanofilaments. Below 853 K, no graphitic structures are formed. Moreover, a new model is proposed for the encapsulation of the catalyst metal particles by graphite layers. (C) 2009 Elsevier B.V. All rights reserved.