Morphology and Growth Region Analysis of Carbon Nanotubes Growth in Water-Assisted Flame Synthesis

Water has been proven to be an effective additive to enhance the growth rate and purity of the synthesized carbon nanotubes (CNTs) in the chemical vapor deposition (CVD) method. Due to similar CNT growth parameters in CVD and flame environment, it is expected that a similar effect can be replicated in CNT flame synthesis to a certain degree. The present study analyzes the effect of water addition to the fuel stream toward the CNT growth in methane diffusion flame under atmospheric conditions. Water vapor was introduced into the fuel gas by passing the methane gas through a water bubbler, and the growth condition was analyzed based on a cross-sectional analysis of the grown CNT on top of nickel wire. The water addition to the fuel reduces the axial extent of the growth region due to the reduction in flame height. Even though the spatial distribution of the growth region in the flame with water additive changes compared to that of the flame without water, the average growth region temperature is relatively identical for both flames at a similar height above the burner. The synthesized CNT morphology did not change with the addition of water due to similar temperatures within the growth region and inhomogeneity of catalyst nanoparticle formation. Remarkably, the thickness of the amorphous carbon layer in the growth region decreases by almost 20% in water-assisted flame due to the reduction of carbon supply caused by the modification of gas-phase chemical reaction and water vapor etching effects that happen within the growth region. Numerical simulation of the flame structure shows that addition of water vapor in the fuel stream lowers the methane concentration within the CNT growth region and simultaneously promotes water etching effects on the amorphous carbon layer.