Mesochannels in two-dimensional hexagonally-ordered mesoporous silica films usually grow parallel to the substrate. However, similar materials with perpendicular channels could potentially be used for applications such as high-density recording media, chemical sensors and highly selective separators. In particular, this ‘standing’ alignment could enhance the incorporation of guest species into the inner parts of the film—a characteristic which may prove useful in some of these applications. The novel properties of ‘standing’ mesostructures prompted Yusuke Yamauchi and colleagues at the National Institute for Materials Science in Tsukuba, Japan,1 to investigate new and simple ways of producing mesoporous films in which the channels are aligned perpendicularly to the substrate.

Their method for synthesizing mesoporous silica film with standing channels involves spin-coating a precursor onto a porous anodic alumina substrate with cone-shaped holes. Notably, the aspect ratio of the conical holes—that is their height divided by their maximum diameter—was found to be an important factor determining whether perpendicular or parallel mesochannels were formed. Low aspect ratios resulted in perpendicular and slightly tilted mesochannels, but at higher aspect ratios, parallel mesochannels were dominant.

Fig. 1: A magnified TEM image of the interface between a mesoporous silica film on a porous anodic alumina substrate. The sponge-like mesopores in the conical holes trigger the formation of tilted, standing mesochannels within the porous silica film.

An ‘oriented-growth mechanism’ is thought to cause this difference in behaviour—in low aspect ratio cones, sponge-like mesopores were formed in the cone, which triggered the evolution of perpendicularly oriented and tilted mesochannels (Fig.1). However, in the higher aspect cones, whilst sponge-like mesopores were present at the apex of the cone, they were replaced by circularly-packed mesochannels closer to the interface of the cone and the film. These doughnut-shaped structures caused the parallel orientation of channels in the films.

“This is a unique approach for controlling the control of the self-organization of surfactants—using their intrinsic mutual interaction and without the application of forced manipulations, such as magnetic or electric processing,” says Yamauchi.

According to Yamauchi, the alignment of the mesochannels is slightly tilted rather than being perfectly perpendicular to the substrate. In the future, the researchers hope to synthesise completely perpendicular mesochannels and to control the pore sizes as well as the thicknesses of the mesoporous films.