Multi-walled carbon nanotubes (MWCNTs) have been grown, simultaneously at different temperatures, by thermal chemical vapour deposition (CVD) of acetylene (C₂H₂) gas, in ammonia (NH₃) atmosphere. The reactor for the CVD growth is a horizontal quartz tube mounted inside a furnace. An accurate calibration of the temperature, at various distances from the tube mean position has been performed by a thermocouple. Silicon oxide substrates, coated with 3 nm of nickel (Ni) film, have been placed at different positions in the reactor and the CNTs have been grown at different temperatures: 750 °C, 725 °C, 700 °C and 600 °C. The tube has been heated by radiation, in an argon (Ar) flux, up to 980 °C (temperature in the middle of the reactor). In order to obtain the Ni nanoclusters, only a NH₃ gas has been introduced into the quartz tube for 20 min at 980 °C. The CNT growth has been performed at 980 °C for 1 hour introducing C₂H₂ and NH₃ gasses with the flux ratio (C₂H₂:NH₃) of 1:5. Scanning electron microscopy (SEM) images show a uniform and a highly dense distribution of CNTs. But, as the growth temperature of substrate increases, the CNT average diameter and density distribution decrease. There is a temperature limit (725 °C) for the CNT growth. At 750 °C we obtain only the Ni clusters with the amorphous carbon. The Raman spectra show that we produce the MWCNTs. The presence of the D-peak indicates that the CNTs have the structural defects to respect to the high crystalline graphitic layers, indicated by the G-peak. Where the temperature of the substrates is higher the D-band line-width is smaller but the ratio of intensities of D and G peaks is still high. The Raman spectra confirm that when the substrate temperature increases the CNT density distribution decreases. The CVD method of growth is totally selective. It is possible to obtain the isolated arrays of CNTs and then to realize the gas sensing (NO₂) devices.

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