Structure of nanoparticle aggregate films built using pulsed-mode electrospray atomization

Abstract

Thin continuous films with tailored micro- and nano-structure were built from nanoparticle aggregates using pulsed-mode electrospray atomization. The aggregates were assembled in-flight and delivered to a target substrate in a dry state. We report on the processing–structure relationship for an individual aggregate and reveal how their assembly determines the structure of the film. Titanium dioxide nanoparticles (P25, ~ 21 nm) were used as a model system given their importance in a wide range of applications. We describe how properties such as nanoparticle concentration, Taylor cone pulsation frequency, solvent volatility, suspension conductivity, and substrate electrical properties govern the size distribution and morphology of the aggregates. We then further report how the aggregate characteristics govern the structure of the film. Aggregates formed using electrospray possess an excess electric charge that can be maintained after they are deposited on the target substrate. This charge accumulation and its dissipation rate play a significant role in film formation. Films are thicker and exhibit a more significant periodic island structure if the dissipation rate of the accumulated charge is enhanced using a conductive substrate or by periodically deactivating the electrospray. Decreasing the separation distance increases the film thickness and island size by limiting the plume dispersion.