Rhombohedral Structure of Highly Ordered and Oriented Self-Assembled Nanoporous Silica Thin Films

Nanoporous silica films have been synthesized by self-assembly on substrates by dip-coating from a combined solution of aged silica oligomers and poly(ethylene oxide) 20 −poly(propylene oxide) 70 −poly(ethylene oxides) 20 triblock copolymer (Pluronic P123) in ethanol and water. The films are indexed with the rhombohedral space group R 3̄ m and are oriented through the thickness of the film with the (111) axis perpendicular to the substrate. Sixfold symmetry perpendicular to the substrate is directly observed by high-resolution top-view field emission scanning electron microscopy over the entire substrate. The films have domains that sample many orientations in the plane of the substrate resulting in a reciprocal space composed of rings. These rings are directly observed by collecting small-angle X-ray scattering data at many angles of incidence from transmission (beam perpendicular to the substrate) all the way down to the grazing angle of incidence. At each rotation, the Bragg spot location in the two-dimensional pattern is quantitatively predicted by an R 3̄ m unit cell with lattice constants a = 112 Å and α = 86°. On the basis of these lattice constants, it is conjectured that the rhombohedral structure results from a uniaxial contraction of a (111) oriented film of self-assembled aggregates ordered on a face-centered cubic lattice. The film structure and orientation reported here differ from previous reports of cubic, or distorted cubic, nanoporous films synthesized with P123 and F127 and are expected to be important steps in controlling structure and accessibility in nanoporous films.