An in Situ Study of Mesostructured CTAB−Silica Film Formation during Dip Coating Using Time-Resolved SAXS and Interferometry Measurements

Abstract

Silica thin films with ordered porosity can be prepared by dip coating, combining polycondensation of silicate species and organization of amphiphilic mesophases. This paper reports on the preparation of templated films using cetyltrimethylammonium bromide (CTAB) as the surfactant and tetraethoxysilane (TEOS) as the inorganic precursor. Depending on the CTAB/Si molar ratio, films with different porous networks 3D hexagonal ( P 6 3 / mmc ), cubic ( Pm 3 n ), and columnar hexagonal ( p 6 m ) were obtained. Identification of the structures was accomplished by coupling two-dimensional X-ray diffraction at grazing incidence and TEM investigations on film cross sections. Other experimental parameters (e.g., deposition rate and composition of the starting sol, including its aging time) appeared also to influence the degree of organization of the final films greatly. To obtain a better understanding of the self-assembly mechanism, the structural formation of the films was followed in real time by in situ SAXS. Because of the very fast self-assembly process (<30 s), high-flux synchrotron radiation was used, which allows to record good quality X-ray patterns in 100 ms. For the first time, interferometry measurements were also combined with the SAXS experiments to correlate the organization steps detected during dip coating with the thickness profiles. A model has thus been proposed for the film formation: organization at the mesoscale (i) takes place at the final stage of liquid-phase evaporation; (ii) involves the formation of micelles at the film/air interface in the early stage of deposition and, in some cases, the formation of intermediate phases; and (iii) is influenced by the condensation degree of inorganic intermediates, the surfactant concentration, and the deposition external conditions.