Modulating Layer-by-Layer Assembly of Oppositely Charged Nanoparticles Using a Short Amphiphilic Molecule

Abstract

Layer-by-layer (LbL) assembly of silica (SiO2) and titania (TiO2) nanoparticles leads to the generation of multifunctional thin films with useful properties. However, favorable film growth for these all-nanoparticle LbL thin films occurs in a narrow pH range. In this work, we widen the processing window of TiO2/SiO2 LbL films by introducing a small amphiphilic molecule, hexylamine (HA), into SiO2 nanoparticle suspensions. Film thickness measurements show that significantly thicker LbL films are obtained after introducing HA into SiO2 nanoparticle suspensions. The zeta potential of SiO2 nanoparticles and that of LbL films are characterized to investigate the role of HA in LbL film assembly. Furthermore, quartz crystal microbalance (QCM) measurements are used to monitor the real-time assembly of LbL films. Our results indicate that HA in SiO2 nanoparticle suspension widens the processing window of LbL assembly by suppressing the negative surface charge of SiO2 nanoparticles and, at the same time, inducing complete charge inversion of LbL films, of which the outermost layer is TiO2 nanoparticles. These two factors lead to the enhanced adsorption of SiO2 nanoparticles and to the formation of TiO2/SiO2 LbL films over a wide pH range. This new approach of using a short amphiphilic molecule is a convenient way of generating all-nanoparticle LbL films over a wide processing window.