Plasma-assisted Synthesis of Carbon Nanomaterial Studied by Spatially-resolved Laser-induced Fluorescence and Optical Emission Spectroscopy*

Crystalline nanographite and carbon nanodots have been of great interest for their applicability in photocatalysis, energy conversion, optoelectronics, and biosensing. In this work, we report a scalable synthesis of sp² carbon nanomaterial consisting of crystalline carbon nanodots and nanographite. This chemical vapor deposition (CVD) approach takes a nonthermal CH₄/Ar capacitively coupled plasma actuated by a 13.56 MHz source in a flow-through tubular reactor. Raman spectroscopy confirmed sp² hybridization with the signature D and G bands. TEM image analysis indicated the average particle size to be ~6 nm.

 

High-resolution optical emission spectroscopy (OES) was used to estimate a gas temperature of ~1200 K. Spatially-resolved planar laser-induced fluorescence (PLIF) was used to capture C₂ species distribution at different locations along the length of the tubular reactor for discharges from different CH₄/Ar ratios. For higher CH₄/Ar ratios, the yield of the sample (collected at the bottom of the tube) is higher and the corresponding C₂ density in the plasma is lower. C₂ density is also found to decrease from 10¹⁸ m^-3 to 10¹⁶ m^-3 down the tube. This can be due to a higher conversion rate of C₂ to nanographite and hence a lower available amount of C₂ to be measured in the plasma. These diagnostics allow for understanding how C_x species evolve from the discharge to form ~6 nm nanoparticles thereby providing for a comprehensive description of the spatiotemporal evolution of graphite nanoparticle growth.