pH-Dependent structure and properties of TiO2/SiO2 nanoparticle multilayer thin films

We demonstrate that the structure and properties of layer-by-layer assembled multilayer thin films comprising positively charged TiO2 and negatively charged SiO2 nanoparticles can be varied by controlling assembly conditions. The average incremental thickness of the all-nanoparticle thin films after a pair of 7 nm TiO2 and 22 nm SiO2 nanoparticle adsorption steps was strongly dependent on the pH of each nanoparticle suspension, varying from a few nanometers to 30 nm. The surface charge density of the adsorbing nanoparticles and that of the previously adsorbed nanoparticle layers are the crucial factors in determining the average bilayer thickness of the films. Zeta-potential measurements of polystyrene microspheres coated with TiO2/SiO2 nanoparticle multilayers indicated that incomplete charge reversal after the deposition of nanoparticles leads to a very small thickness increase at each adsorption step, whereas complete charge reversal results in significantly larger thickness increments. Other film characteristics including refractive index, porosity, and chemical composition of the multilayers also depended on assembly conditions but to a much lesser extent compared to the dependence observed for average bilayer thickness.