Laser-Induced Emission of TiO2 nanoparticles in Flame Spray Synthesis

Flame spray pyrolysis (FSP) is considered a powerful method to produce nanoparticles oxides with peculiar and tailored properties, thanks to the possibility to properly select flame experimental conditions. Particle size is a key parameter in nanopowder characterization since particle properties are often related to surface-to-volume ratio. For that, the development of a tool able to follow nanoparticle size during formation and growth in the flame synthesis is of great interest. Laser-induced incandescence technique is widely used to provide such information. In this work, the applicability of the technique to TiO2 nanoparticles in flame spray synthesis is explored. The flame spray available in the ICMATE laboratory in Milano is used to produce nanoparticles in powder or deposited on a proper substrate. In our experimental conditions, the particles present a diameter lower than 20 nm. The flame under analysis is characterized by a high adiabatic flame temperature and consequently by a significant emission radiation, which makes particularly challenging the incandescence measurements. Moreover, fluorescence emission from titania nanoparticles can strongly influence the incandescence as well. In order to investigate the feature of the incandescence signal and the interference with fluorescence, preliminary time-resolved spectral laser-induced incandescence measurements are carried out on TiO2 nanoparticles deposited on a glass fiber filter in different laser fluence regimes. Then, spectral measurements are carried out on the flame spray at different heights above the burner and changing the acquisition delay time. The analysis proves the applicability and the challenges in using this diagnostic tool in flame spray synthesis.