In situ Raman characterization of nanoparticle aerosols during flame synthesis

Raman spectroscopy is applied to diagnose nanoparticle presence and characteristics in a gaseous flow field. Specifically, in situ monitoring of the Raman-active modes of TiO₂ and Al₂O₃ nanoparticles in aerosol form is demonstrated in high-temperature flame environments. This technique serves as a sensitive and reliable way to characterize particle composition and crystallinity (e.g. anatase versus rutile) and delineate the phase conversion of nanoparticles as they evolve in the flow field. The effect of temperature on the solid-particle Raman spectra is investigated by seeding nanoparticles into a co-flow jet diffusion flame, where local gas-phase temperatures are correlated by shape-fitting the N₂ vibrational Stokes Q-branch Raman spectra. Applying the technique to a flame synthesis environment, the results demonstrate that in situ Raman of as-formed nanoparticles can be readily applied to other gas-phase synthesis systems, especially as an on-line diagnostic.