Self-organized macrostructures in anodically formed mesoporous silica

The electrochemical oxidation of silicon under high potentials (10-30 V) was investigated in weakly alkaline low-buffered fluoride media. The cyclic voltammogram exhibits a hysteresis of several orders of magnitude. Potentiostatically grown oxide films are either pale gray or show thin-film interference colors. Optical spectroscopy indicates here a porous silica layer on a thin compact SiO₂ film. Electron microscopy shows that the brilliant films consist of granular SiO₂ particles with sub-100 nm diameters with a homogeneous film thickness over the etching area. For U > 15 V micrometer-sized corrugations lead to a gray appearance. These macrostructures self-organize in a close-packed lattice with narrowly distributed dimensions.