The catalytic growth of high aspect ratio carbon nanorods by ethylene decomposition over nickel/silica is presented as a viable low-cost selective route to a high-value product. This study focuses on the role of catalyst preparation in determining carbon yield and structural characteristics and considers the application of a 10% w/w Ni loading prepared by impregnation and precipitation/deposition. The latter is characterized by a narrower dispersion of smaller Ni particles (average diameter = 2.4 nm) but lower carbon yields. Doping this catalyst with KBr resulted in a 40-fold increase in carbon production with >95% having rod diameters <10 nm; doping the impregnated catalyst had little effect on catalyst performance. High-resolution transmission electron microscopy and temperature programmed oxidation were employed to characterize the catalysts and the carbon product; the effect of temperature (673-873 K) on carbon yield is also reported.