In this work, for the synthesis of CNT arrays, a one-step heat-treated aluminum foil substrate-supported Fe(NO₃)₃ solution “self-reactive process” was developed to prepare Fe@Al₂O₃ nanoparticles with a diameter of about 20 nm and a catalyst layer thickness of about 2–3 mm. The results show that the key to maintain the small nanoparticle state is the interfacial interaction between the metal and the metal oxide, which enables the Fe nanoparticles to anchor on the Al₂O₃ surface and effectively prevents the aggregation or segregation of the Fe nanoparticles. Furthermore, by comparing the characterization parameters of the catalyst nanoparticles prepared under a series of different conditions, it was found that the aluminum foil substrate was oxidized to Al₂O₃ by the Fe(NO₃)₃ precursor in a one-step heat treatment, which could be explained by the oxidation of nitrate ions. Meanwhile, the core-shell structure formed by some of the Al₂O₃ particles with the catalyst prevented the agglomeration of the catalyst during the reduction process by co-deposition of metal oxides with Fe catalyst. This method saves a lot of time compared with other catalyst preparation methods for growing CNT.