Mostafa Bedewy1,Golnaz Najaf Tomaraei1,Jaegeun Lee1,2,Moataz Abdulhafez1
University of Pittsburgh1,Pusan National University2
Mostafa Bedewy1,Golnaz Najaf Tomaraei1,Jaegeun Lee1,2,Moataz Abdulhafez1
University of Pittsburgh1,Pusan National University2
A key challenge in controlling the height of CNT forests grown in conventional chemical vapor deposition (CVD) reactors arises from the fact that the temperatures of catalyst formation, <i>T<sub>c</sub></i>, and CNT nucleation and growth, <i>T<sub>g</sub></i>, are coupled. Increasing this coupled temperature results in a trade-off between an increase in the growth rate and a decrease in the catalytic lifetime due to the acceleration of catalytic deactivation.<br/>Commonly known deactivation mechanisms include carbon overcoating, chemical poisoning, mechanical coupling, and catalyst loss by migration on the surface or to the underlying layer. These mechanisms depend on the conditions of both catalyst formation and growth stages. Catalyst formation stage involves solid-state thin film dewetting in a reducing environment, which determines the initial chemical state, number density, and size distribution of catalyst nanoparticles. The chemical and physical evolution of catalyst nanoparticles continues during CNT growth stage until catalytic deactivation. We use a custom-designed CVD reactor, composed of a preheater for decoupling gas phase decomposition temperature and a multizone rapid thermal processing (RTP) furnace for the decoupling of <i>T<sub>c</sub></i> and <i>T<sub>g</sub></i>. The RTP furnace allows us to rapidly change the temperature after catalyst formation stage, right before CNT growth stage. Revealing the decoupled effect of <i>T<sub>c</sub></i> on forest height and density, especially when the effect of gas phase decomposition temperature is also independently controlled, enables unprecedented process control.<br/>Our findings show that increasing <i>T<sub>c</sub></i> at any given <i>T<sub>g</sub></i> results in a taller CNT forest, while increasing <i>T<sub>g</sub></i> at constant <i>T<sub>c</sub></i> reduces density and increases growth rate. Real-time growth kinetics reveal that the increase in forest height (for constant <i>T<sub>g</sub></i>) is due to an increase in catalytic lifetime, while the growth rate remains unaffected (with changing only <i>T<sub>c</sub></i>). Atomic force microscopy (AFM) analysis provides evidence that catalyst loss by subsurface diffusion is suppressed at higher <i>T<sub>c</sub></i>. Full characterization of alumina support layer by X-ray diffraction, ellipsometry, nanoindentation, and surface energy measurement show that increasing <i>T<sub>c</sub></i> to 900 °C leads to the evolution of alumina into a denser and less porous film with higher crystallinity and Lewis basicity. Therefore, our decoupled approach is capable of increasing catalytic lifetime by tailoring the properties of the oxide support layer in a single rapid thermochemical pretreatment step in such a way that suppresses the subsurface diffusion of catalyst nanoparticles into the support layer.