Nanowire Forests Deposited via Homogeneous Gelation in Electrospray Deposition

In recent years, electrospray deposition (ESD) has gained increasing attention in nanotechnology with its unique ability to produce monodispersed droplets in nano/microscale. The morphology of coatings created by ESD be generally divided into three categories: wire mats, particles, and films. The deciding factor in the formation of these morphologies is the competition between the instantaneous viscosity of the solution and the surface forces during the process. At intermediate regimes, there should exist nanowire forests as a mixture of wire and particulate deposition, but high aspect ratio (AR) particles have been rarely observed in ESD. We have revealed through electrostatic dissipative particle dynamics (DPD) simulations that the barrier to forming nanowire forests is the directional nature of evaporation, implying that they would form were evaporation homogeneous. Experimentally, ESD was utilized to explore the spray of a fibril gel former, methylcellulose (MC) in water:ethanol blended solutions. MC possesses a lower critical solution temperature (LCST) in water and water:ethanol blends. Above the LCST, MC and water phase separate concurrently with the rapid evaporation of ethanol, forming a shear-thickening, homogeneous gel phase. In the ESD process, this gelation is enhanced by shear and surface charge and electrospins on a drop-by-drop basis to create forests of individual nanowires. We explored altering the gel properties through changes in (1) molecular weight, (2) loading concentration, (3) flow rates, (4) spray distance and (5) additive contents. The results indicate that lowering the viscosity of the gel increases the AR of formed nanowires, with optimized conditions leading to wires of AR 18. Similarly, the incorporation of additives that lead to the changes of viscosity controlling the formation of nanowires in ESD. For example, addition of poly(ethylene glycol) (PEG) as a plasticizer allowed for a 50 wt% increase from AR 10 to AR 18 for the same spray conditions through simultaneous reduction in the wire diameter and increase in the wire length. Our study indicates that the homogenous evolution of viscosity is necessary for nanowire forest formation and that the specific viscosity controls the morphology of the forests. We also demonstrated that these wires with and without PEG modification can be used to encapsulate gold nanoparticles. This allowed for hierarchical plasmonic coatings that maintained the single-particle plasmon at up to 83 wt% loading of particles by preventing agglomeration. In addition, we employed a hole depth array approach to confirm that the ESD of nanowires is a self-limiting electrospray deposition (SLED),  which provides the possibility of manufacturing functional cilia-like structures scalably on complex 3D surfaces.