Growth region characterization of carbon nanotubes synthesis in heterogeneous flame environment with wire-based macro-image analysis

Abstract

Identification of carbon nanotube (CNT) growth region is paramount in a heterogeneous environment of flame to ensure an improved yield and growth rate. Previously, the growth region analysis in flame synthesis has been done based on repetitive scanning electron microscope (SEM) characterization that tends to be costly and time-consuming for a large heterogeneous environment. A systematic evaluation of the experimental method for growth region characterization is very limited. Therefore, the present study provides a detailed sensitivity and accuracy evaluation of a simple wire-based macro-image analysis (WMA) method for the measurement of steady-state and temporal change of growth region size for multiwalled carbon nanotubes (MWCNT) in a methane diffusion flame with varied air flow rate at atmospheric condition. The WMA method was developed to simplify the MWCNT deposit region identification using the image produced by digital single-lens reflex (DSLR) camera and post-processed using baseline data that was gathered through a one-time SEM analysis. Pure nickel wire was positioned in the flame with a stainless-steel wire grid placed on top of the substrate wire to redistribute the flow field. The MWCNT deposit region that is confined in the fuel-rich region at the inner side of the flame sheet shifts toward the flame centerline with the increase of the air flow rate from 1 to 10 slpm due to the shift of the flame sheet in the same direction. The inception and growth of MWCNT are consistently observed together with the formation of amorphous carbon layer, which has been verified based on detailed SEM analysis. The formation of amorphous carbon layer happens due to the higher rate of carbon supply compared to that of the carbon diffusion rate in nickel catalyst for CNT growth. These phenomena serve as the fundamental basis of the present WMA method. At 10 mm above the burner, the WMA method at steady state successfully detected the change in size and spatial distribution of deposit region by a factor of 2.28 and 2.68 respectively at varying air flow rate with ± 0.5 mm accuracy. The present method demonstrated high sensitivity to capture the temporal change of the deposit region length within the exposure time for the growth rate measurement of the deposit region. More than twofold increase in effective growth rate was successfully captured at a high air flow rate due to the change in equivalence ratio from rich to stoichiometry that promotes the increase in heat release rate. The present flame setup was able to produce a widest CNT deposit region with an optimum growth rate at 6 slpm air flow rate. High-resolution transmission electron microscopy (HRTEM) analysis verified that the synthesized CNT is hollow with multiwalled internal structure and Raman spectroscopy analysis indicated that crystallinity level of the produced MWCNT is relatively constant with the change in air flow rate.