The synthesis of aligned multi-walled carbon nanotubes (MWCNTs) using thermal and floating catalytic chemical vapor deposition (CVD) method has been optimized in order to obtain MWCNTs with specific characteristics namely diameter and thickness of nanotubes array. Process parameters such as substrate preparation which involved buffer layer deposition, temperature and reaction duration were studied. Samples produced were analyzed using FESEM, HRTEM and Raman spectroscopy. Typical thickness of CNTs array obtained using thermal CVD is 38 μm whilst the ones from the floating technique have a wide range of thickness with the thickest being about 639 μm for the duration of 1 hour. Floating CVD method has the capability to produce good quality, aligned CNTs array with various thicknesses required to vary the electrode gap of the ionization-based gas sensor for the reduction of the breakdown voltage, leading to low power consumption and safe operation of the sensor. © (2013) Trans Tech Publications, Switzerland.