Use of Mixed Langmuir Films of Nanoparticles to Form Metal Oxide Materials with the Optimal Surface Charge

We aimed to prepare metal oxide materials with the optimal surface charge by preparing mixed films of non-modified metal oxide nanoparticles (NPs) with dissimilar isoelectric points (iep). The purpose of preparing such surfaces was to expand the use of metal oxide materials in environments where the solution pH cannot be changed. Langmuir films of SiO₂ (iep: pH 2-3) and TiO₂ (iep: pH 5-6.6) NPs were first prepared at air-100 mM NaCl aqueous interfaces. This subphase allowed the formation of stable films of the NPs without the need to modify the NPs with surface-active chemicals, whose presence may detrimentally change the properties of the films. The Langmuir films were then transferred and sintered to silicon wafers and their physical properties were characterized using atomic force microscopy (AFM). The AFM images showed that the films were composed of NP aggregates. The average size of the aggregates decreased, and the uniformity of the aggregate sizes and their inter-aggregate spacing increased with the addition of SiO₂ NPs to the film of TiO₂ NPs. These changes were explained by an increased electrostatic and steric repulsion between the aggregates formed at the air-100 mM NaCl interface due to the adsorption of negatively charged SiO₂ NPs to the slightly positively charged TiO₂ aggregates. The force-distance curves measured between a SiO₂ probe and the sintered films of SiO₂ and/or TiO₂ NPs in a 1.0 mM NaCl solution adjusted to pH 4 showed that the magnitude of the repulsive force decreased with an increased number of TiO₂ NPs in the film. This force change indicated that the surface charge changed when different types of NPs were mixed. These results indicate that mixing different NP types in a Langmuir film at an air-aqueous interface can help change the physical properties of the transferred film.