Catalyst Design for the Synthesis of Vertically Aligned and Double-Walled Carbon Nanotube Array Exceeding One-Centimeter Height

Since metal nanoparticle catalysts often face challenges of catalytic degradation due to structural changes, the design of the catalyst support layer is crucial, especially for the synthesis of vertically aligned carbon nanotubes (CNT forests) from highly dense catalytic nanoparticle arrays [1-3]. Our previous research, which primarily focused on the impact of catalyst support, emphasizes the importance of the double-layer (DL) strategy for the catalyst support layer. DL supports, composed of a top “anchor layer” to impede surface diffusion such as Ostwald ripening, and a high crystallinity “sealing layer” to inhibit subsurface diffusion, have been proposed and demonstrated to elongate the catalyst lifetime for the synthesis of mm-tall single-walled CNT forests [4].
In the present study, we explored the DL catalyst support structure and successfully demonstrated the selective synthesis of DWCNT forests exceeding 1 cm in height in just 60 minutes. The catalyst support was prepared by sputtering an alumina layer (thickness: approximately 1−3 nm) onto pre-annealed SiO₂ layer formed on Si substrate. Sequentially, an iron catalyst film was sputtered onto the catalyst support. The catalyst was then annealed in hydrogen to form catalyst nanoparticles, followed by CNT synthesis using water-assisted catalytic chemical vapor deposition. Observations using an atomic force microscope (AFM) revealed the formation of iron nanoparticle arrays with a relatively low particle density of 3×10¹¹ particles/cm² after hydrogen annealing. Structural evaluation of the CNT forest revealed a sparse array (0.8×10¹¹ tubes/cm²) of mainly double-walled (56%) CNTs.

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