Controlled mesoporous film formation from the deposition of electrosprayed nanoparticles

Abstract

Thin films of controlled morphology were fabricated by electrospray drying a colloidal nanoparticle suspension using a conductive and volatile solvent and impacting the nanoparticles on a substrate. Three parameters were used for control: impact velocity, size of the nanoparticles or nanoparticle agglomerates, and solvent volatility. The impact velocity was controlled by charging nanoparticles through electrospray dispersion and varying the electric field driving the particle impaction. It was found that the structure is governed by the relative importance of charged particle drift imposed by the external electric field and the thermal velocity due to Brownian motion. Peclet number correlates with the morphology of the deposit where columnar structures result from high Pe, corresponding to ballistic deposition and porous, fractal-like structures result from small Pe. These patterns match predictions based on Monte Carlo simulations in the literature. For dispersions with higher nanoparticle concentrations, droplet evaporation causes densification of the particle ensemble to form a spherical aggregate that deposits in a predominantly ballistic manner, with smaller aggregates forming denser films. If the droplet evaporation lifetime is altered for the aggregates to be partially wet upon impacting the substrate, the subsequent rapid evaporation of the remaining solvent on the substrate leads to formation of films with high interconnectivity. Films formed by the electrospray technique have large-scale uniformity and their structure is independent of thickness. The interpretation of the observed morphologies in terms of Peclet number and Damkh€ oler number provides a conceptual framework for a rational design of film structures as required by many applications.

A comparative study on droplet characteristics and specific charge of ethanol in two small-scale electrospray systems

“…For example, it can yield quasi-monodisperse droplets, control the droplet motion easily and avoid coalescence of droplets 1 . It has been applied in many areas such as fuel atomization in combustion systems 2,3 , controlled droplet generation 4,5 , electrospinning 6,7 , thin film deposition 8,9 and electrospray ionization-mass spectrometry applications 10,11 . Meanwhile more and more attention has been focused on the micro-power systems using micro-scale combustors due to the higher specific energy of liquid hydrocarbons than that of batteries 12,13 .…”

Electrospray: More than just an ionization source

“…This is particularly useful in the production of thin nanoporous films and coatings, which can be additionally structured into well-defined arrangements and geometries. The chemical composition of processed nanoparticles is determined by the target application and can include silver [75,189] (surface-enhanced Raman spectrometry), iron oxides [39] and iron [190] (recording media, sensors), carbon [191] (electrodes), titanium oxide [189,192,193] (gas sensors, photocatalysis), gold [43,194,195] (hybrid memory elements), silicon oxide [195,196], polystyrene [189,197], and many other materials. Most of these applications require the coating to have uniform properties across the entire treated surface; therefore, cone-jet mode is typically employed as it produces a fine aerosol with a narrow size distribution of droplets [43].…”

“…For example, Castillo et al increased the deposition efficiency of positively charged nanoparticles by applying a negative voltage to the substrate, which was placed on a grounded collector [191]. Similarly, introducing an auxiliary electrode between the emitter and the substrate significantly improved control over the trajectories of the nanoparticles [193,199].…”

“…In the context of film production, the in-plume drying process is of utmost importance. Any residual solvent can cause the annealing of structures and/or the coffee-ring effect, both of which negatively affect the homogeneity and porosity of the coating [191,193,198].…”

Ba0.6Sr0.4TiO3 Thin Films Deposited by Spray Coating for High Capacitance Density Capacitors

“…A low evaporation rate (low substrate temperature) will allow the spreading of droplets that will form a wet film. The Brownian motion of nanoparticles enhanced by the temperature will lead to the formation of large agglomerates at the surface of the film . On the other hand, a high substrate temperature ( T = 250 °C) will hinder droplets spreading on the substrate and lead to films with a very few concentration of agglomerates at the surface (Figure c and e).…”