Toward Dynamic Control over TiO2 Nanocrystal Monolayer-by-Monolayer Film Formation by Electrophoretic Deposition in Nonpolar Solvents

The controlled electrophoretic deposition of monolayers and ultrathin films of 4.0 nm TiO2 nanocrystals from stable, nonpolar solvent-based suspensions is reported. Stable suspensions were prepared in hexane, and the electrophoretic mobility of the nanocrystals was enhanced by a combination of a liquid liquid extraction followed by mechanical surfactant removal by high-speed centrifugation. The controlled evolution of the density of TiO2 nanocrystal monolayers was studied by transmission electron microscopy and optical transmittance spectroscopy. Ultrathin films were assembled while maintaining monolayer-by-monolayer growth and uniform density of the film. A time-dependent, equivalent circuit model has been proposed to characterize the electrophoretic current that was recorded during our experiments. Further, we demonstrate that the proposed model, coupled with the mobility, provides a means to estimate the deposition rate and, hence, the time necessary to fabricate a submonolayer, a monolayer, and multilayers of nanocrystals.