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[Display omitted] • The single-walled carbon nanotubes and multi-walled carbon nanotubes were grown on the same ZrO 2 /Fe catalyst by adjusting the carbon sources. • A combination of experimental data and density functional theory simulations was able to reveal the underlying activity competition mechanism between hyperactive catalysts and hypoactive supports. Activity competition commonly exist when using multiple catalyst particles for growing carbon nanotubes (CNTs), and the diverse catalysts can compete with the carbon source and affect the growth kinetics and morphology of the nanotubes. However, the existed problem is that whether the support materials themselves also can engage in activity competition with the catalysts, and how this may impact the growth of CNTs. Here, we investigated the influence of hypoactive ZrO 2 support material and hyperactive ZrO 2 /Fe catalysts on the growth of CNTs, and the activity competition relationship between the support and the catalysts. ZrO 2 /Fe catalysts were prepared via a facile hydrothermal method, and employed both CH 4 and C 2 H 5 OH as carbon sources for growing CNTs. It is interesting that single-walled carbon nanotubes (SWCNTs) were grown by utilizing CH 4 as carbon source, and multi-walled carbon nanotubes (MWCNTs) were synthesized for the use of C 2 H 5 OH as carbon source. A combination of experimental data and density functional theory simulations was able to reveal the underlying activity competition mechanism between hyperactive catalysts and hypoactive supports. It would provide new insight into the growth mechanisms of CNTs, and provide new ideas for the rational design and controlled growth of CNTs. [ABSTRACT FROM AUTHOR]