Two-dimensional hexagonal mesoporous silica thin films prepared from block copolymers: Detailed characterization and formation mechanism

Silica thin films with a two-dimensional (2D) hexagonal structure were produced from TEOS and the F127 triblock copolymer PEO₁₀₆PPO₇₀PEO₁₀₆. The structure of the films was deduced from transmission electron microscopy (TEM) performed on microtomed sections and from a 2D X-ray scattering technique adapted for film characterization. Both methods are complementary and allow for a detailed characterization of the film structure. The pore channels within the coatings are organized in domains, which spin and curve inside the film, but align with the surface plan at the air/film and film/substrate interfaces. The formation of the film was monitored by in situ time-resolved SAXS experiments to follow the structural evolution during the first minutes after deposition. Cylindrical micelles, which gives rise to a wormlike structure, form first at the air/film interface and extend toward the film/substrate interface via solvent evaporation. When most of the liquid phase is evaporated, the well-aligned domains located at both interfaces are obtained by a self-arrangement of the micelles parallel to the interfaces. This rearrangement occurs when the film is considered to be dry. The atom content profiles in the as-prepared films were obtained by Rutherford backscattering (RBS) and show homogeneous distribution of atoms throughout the film thickness. To stiffen the network and remove the surfactant, various postsynthesis treatments were applied to the coatings, that is, thermal treatment, base-catalyzed condensation, and solvent extraction. Each method induces shrinkage, leading to 2D-centered rectangular films through a contraction of the initial 2D hexagonal structure. As-prepared and treated films exhibit excellent optical transparency and high degree of organization. Ellipsometry measurements and N₂ adsorption/desorption isotherms were thus used to estimate the refractive index and the porosity of the films. Refractive index down to 1.2 was obtained in the film that was pretreated under a NH₃ atmosphere before solvent extraction of the surfactant. This shows the efficiency of this treatment to minimize the network shrinkage. N₂ adsorption-desorption measurement performed on films calcined at 350 °C gives a surface area of 800 m² g^-1 and a porosity of 63%.