Self-Assembly of Metal Oxide Nanosheets at Liquid–Air Interfaces in Colloidal Solutions

The oxide nanosheet concentration at the liquid–air interface (LAI) is a key parameter in the formation of Langmuir–Blodgett (LB) deposited nanosheet films. Knowledge of the oxide nanosheet concentration at the LAI as a function of process conditions is needed to understand the relevant processes and achieve better control over the LB fabrication process. In this study, the concentration of Ti_0.87O₂^δ− titanate nanosheets at the LAI was investigated by considering the trend in the lift-up point (LUP) in the surface pressure–surface area isotherm of an LB compression process as a function of time and exfoliation agent. The oxide nanosheet concentrations in the bulk solutions were studied using UV–vis spectroscopy. The results show that the restacking process in the bulk solution does not significantly retard the occurrence of nanosheets at the LAI. The nanosheet concentration changes in the bulk and at the LAI occur on different time scales. Short exfoliation times yield higher nanosheet concentrations at the LAI than longer exfoliation times, in contrast to the bulk where the nanosheet concentration increases in the course of time. We found the same behavior for other metal oxide nanosheet solutions, i.e., iron-doped titanate (Ti_0.6Fe_0.4O₂^0.4–) and calcium niobate (Ca₂Nb₃O₁₀^–) nanosheets. The reason behind this phenomenon is likely related to the high degree of adsorption of surfactant molecules on the nanosheet surface after short exfoliation times