The true structure of hexagonal mesophase-templated silica films as revealed by X-ray scattering:: Effects of thermal treatments and of nanoparticle seeding

This work describes the detailed structural investigation of mesophase-templated mesoporous silica films by 1D and 2D X-ray scattering techniques and transmission electron microscopy. The films are prepared by sol-gel dip coating with 2D hexagonal templating mesophases, yielding 2D mesoporous structures consisting of cylindrical pores whose axes are aligned parallel to the surface. It is shown that drying and thermal treatments induce an unidirectional shrinkage of the layers in the direction of the normal of the film. The true rectangular symmetry is only evidenced by 2D X-ray scattering in two different scattering geometries. 1D diffraction gives only an apparent hexagonal symmetry. It is furthermore shown that although the cylinder axes are randomly orientated within the plane parallel to the surface, there are large domains with well aligned 2D planar unit cells perpendicular to the surface. It is demonstrated that this preferential ordering is destroyed by nanoparticle seeding with amorphous silica or maghemite particles.