Raman Spectroscopy On Size Segregated Nanoparticles From a Flame Generator

Flame methods are widely used among nanoparticle synthesis via aerosol routes. Many flame methods include spraying of liquid precursor followed by evaporation and nucleation into product species. Liquid Flame Spray (LFS) works in this way. The precursor solution is sprayed into a turbulent, high temperature, high velocity H₂-O₂ flame. Final particle size is adjusted with the precursor concentration and feed rate. A characteristic of these spray methods is that the sprayed droplets are not completely evaporated and so called residual particles are formed. Mainly the synthesised nanoparticles form via gas-to-particle conversion. However, the slightly larger residual particles are formed via liquid-to-solid conversion. This might lead to different crystalline forms. Conventional way to study the crystalline forms is X-Ray diffraction from powder samples. However, it is only an average of the whole sample. In this study, we combine aerosol sampling in impactor with Raman spectroscopy in a novel way. We synthesise nanoparticles with the LFS and collect the particles in a cascade impactor.  Different size fractions are segregated in the impactor. Afterwards, the impactor plate samples are studied using Raman spectroscopy to distinguish the crystalline form of different parts in the aerosol distribution. Iron oxide and titanium dioxide particles are studied separately. Iron oxide is found to be mainly magnetite but the residual particles are hematite. Titanium dioxide is anatase as produced and there can be found residual particles of rutile.