Two-dimensional hexagonal mesoporous silica thin films prepared from blackcopolymers: Detailed characterization amd formation mechanism

Silica thin films with a two-dimensional (2D) hexagonal structure were prod uced from TEOS and the F127 triblock copolymer PEO106PPO70PEO106 The struct ure of the films was deduced from transmission electron microscopy (TEM) pe rformed on microtomed sections and from a 2D X-ray scattering technique ada pted for film characterization. Both methods are complementary and allow fo r a detailed characterization of the film structure. The pore channels with in the coatings are organized in domains, which spin and curve inside the f ilm, but align with the surface plan at the air/film and film/substrate int erfaces. The formation of the film was monitored by in situ time-resolved S AXS experiments to follow the structural evolution during the first minutes after deposition. Cylindrical micelles, which gives rise to a wormlike str ucture, form first at the air/film interface and extend toward the film/sub strate interface via solvent evaporation. When most of the liquid phase is evaporated, the well-aligned domains located at both interfaces are obtaine d by a self-arrangement of the micelles parallel to the intel faces. This r earrangement occurs when the film is considered to be dry. The atom content profiles in the as-prepared films were obtained by Rutherford backscatteri ng (RBS) and show homogeneous distribution of atoms throughout the film thi ckness. To stiffen the network and remove the surfactant, various postsynth esis treatments were applied to the coatings, that is, thermal treatment, b ase-catalyzed condensation, and solvent extraction. Each method induces shr inkage, leading to 2D-centered rectangular films through a contraction of t he initial 2D hexagonal structure. As-prepared and treated films exhibit ex cellent optical transparency and high degree of organization. Ellipsometry measurements and Nz adsorption/desorption isotherms were thus used to estim ate the refractive index and the porosity of the films. Refractive index do wn to 1.2 was obtained in the film that was pretreated under a NH3 atmosphe re before solvent extraction of the surfactant. This shows the efficiency o f this treatment to minimize the network shrinkage. NP adsorption-desorptio n measurement performed on films calcined at 350 degreesC gives a surface a rea of 800 m(2) g(-1) and a porosity of 63%.