Limitation in growth temperature for water-assisted single wall carbon nanotube forest synthesis

Abstract

In this study, we examined the limitations in growth temperatures for single-walled carbon nanotube (SWNT) forest synthesis using a water (H₂O) growth enhancer as highlighted by a dramatic decrease in growth efficiency at high temperatures. Comparative synthesis using a carbon monoxide (CO) growth enhancer demonstrated a wider temperature window for SWNT forest synthesis. We found that SWNT forests taller than 100 μm could not been synthesized above 850 °C by using H₂O, but by using CO, tall (>800 μm) forests could be synthesized at growth temperatures exceeding 900 °C. The dependency of H₂O concentration showed that H₂O, itself, was the cause for the reduced growth efficiency observed at higher temperatures. In contrast, increased CO concentrations did not result in any drop in the carbon nanotube (CNT) yield across the entire temperature range. While a severe drop of CNT yield above 950 °C was observed when CO was used, the origin was found to stem from catalyst particle aggregation enhanced by diffusion on the surface rather than CO itself. Therefore, the ability to synthesize at higher growth temperatures is advantageous when using more stable carbon feedstocks, such as methane.