It is Not the Catalyst: How Angle-of-Attack Engineering Overcomes Diffusion Limits in CNT Forest Growth

Vertically aligned carbon nanotubes (VACNTs) combine high specific surface area, excellent thermal and electrical conductivities, and tunable directionality, making them attractive for applications including water filtration, energy harvesting, biosensing, and thermal interfaces. Maximizing VACNT height is critical to reduce CNT−CNT junction density and increase accessible surface area. While VACNT height is typically optimized through catalyst design and gas composition, reactor-scale fluid dynamics also strongly influence precursor transport. Here, first-order computational fluid dynamics (CFD) simulations are coupled with growth experiments to clarify how gas-phase transport limits attainable VACNT height and to identify strategies to delay self-limiting growth. Simulations reveal that increasing forest height leads to thickening of the flow boundary-layer (BL) along the substrate. With a Schmidt number near unity, the mass diffusion BL scales similarly, reducing reactant flux to the catalyst and inducing diffusion-limited growth. To mitigate this effect, substrates were modeled at positive angles of attack (10°−30°) relative to incoming flow, which consistently reduced BL thickness across all heights. Guided by these results, substrates were mounted at 15°−25° during in situ CVD growth for durations of 8−26 min. Experiments show that relative height enhancement is most pronounced for shorter forests (up to ∼40%) and decreases for taller ones (∼15%), consistent with BL-mediated limitations. These findings demonstrate that passive geometric control of substrate orientation alleviates transport constraints, increasing VACNT height without altering growth chemistry and shifting optimization from catalyst-centric models to those incorporating reactor-scale transport effects.