Droplet generation and nanoparticle formation in low-pressure spray pyrolysis

In the present study, we found that pressure-dependent droplet-to-particle formation is important for the control of both droplet and particle characteristics during the production of particles with enhanced properties. Using experimental and simulation approaches, the present study investigated the droplet-to-particle formation of zinc oxide as a model material that was synthesized via low-pressure spray pyrolysis. The size distributions of both the droplets generated under low-pressure and the synthesized particles were measured and compared. The effects of operating conditions, i.e., operating pressure, carrier gas-, and liquid-flow rates, reactor temperature, and precursor concentration, on the characteristics of the generated droplets and synthesized zinc oxide particles were systematically investigated. We found that broad and bimodal droplet size distributions shifted to narrow and monodispersed distributions as chamber pressures were reduced. The low-pressure-synthesized zinc oxide nanoparticles were highly crystallized with unimodal distributions and sizes of approximately 8 nm. Droplet temperature decreased with decreasing chamber pressure, which confirmed both the evaporative cooling of droplets and the formation of smaller droplet sizes. The chamber temperature and droplet size simulation results were in accordance with the experimental results. In conclusion, we found that the chamber pressure is the primary determinant of monodispersity with respect to droplet size distribution and to the size and morphology of the synthesized particles.