Metal oxide nanoparticles formed from solution droplets under high heating rate

The formation mechanisms of combustion generated metal oxide nanoparticles were explored in a stoichiometric laminar premixed flame doped with droplets of cadmium, nickel(II) and lead(II) nitrate aqueous solutions. Generated particles were thermophoretically collected and analyzed by Atomic Force Microscopy (AFM). The results showed that most of the particles have sizes lower than 10 nm. The size distribution function shapes and time evolutions depend on the metal salt solubility and thermal decomposition characteristics. By comparing the thermophoretically collected matter and the amount of injected metal precursors, a size dependent adhesion efficiency of the particles on probe mica plates has been found. The results showed that nanoparticles have a low capability to adhere on a surface, regardless of the used metal. The adhesion efficiency quickly decreases for particles smaller than 10 nm. As a consequence, the smallest particles are present in the flame with a relative high number concentration. This feature is of great interest when developing filtering systems able to remove nanoparticles with size lower than 10 nm at the exhaust of combustion systems.

► Water solution droplets of metal nitrates were suddenly heated in a premixed flame.
► Cadmium, nickel and lead oxide nanoparticles (D > 1 nm) were generated and measured.
► Nanoparticles sizes depends on solubility and thermal decomposition of nitrates.
► Particle adhesion efficiency on substrates rapidly decreases for sizes below 10 nm.
► Adhesion is mainly affected by particle sizes as opposed to their chemical nature.