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Advanced Coatings & Surface Technology covers the latest innovations and
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Sample Briefing
FABRICATE MESOPOROUS SILICA FILMS WITH THROUGH CHANNELS
Mesoporous materials are extremely interesting because they have potential
applications as high-performance catalysts, catalytic supporters, and nanoscale
cages for constructing magnetic and electronic devices. Ideally, such films have
one-dimensional pores that are perpendicular to the substrate.
Silica thin films made by liquid crystal templating of zeolite powders are
promising candidates, and researchers have made many attempts to synthesize such
films with a controlled pore orientation, but with poor results. The films
either have pores that grow parallel to the substrate or have 3D structure.
Shinji Kondoh and his colleagues at the Fine Ceramics Research Association in
Japan have developed a novel processing technique for fabricating mesoporous
silica films that have one-dimensional through channels that are perpendicular
to the substrate surface. They got the idea for the method from the study of
oxidation of the silicate mineral fayalite, or Fe2SiO4. When heated in air, Fe
oxidizes and loses an electron, changing from 2+ to 3+. The result is
decomposition into a regular array of platelike magnetite crystals and amorphous
platelike silica with plate widths about 50 nm. More interesting is that the
plates elongate parallel to the direction of oxygen diffusion, independent of
the original fayolite crystal orientation.
In order to form films that mimic the microstructure of preferentially
oxidized fayolite, Kondoh and his colleagues developed a three-step process that
involves chemical etching. It starts by rf magnetron sputtering (using argon) an
amorphous membrane of Fe-Si-O on silica glass at constant power of 4.4 W/cm2 and
pressure of 2 Pa. Then the membrane is oxidized by heat treating at 600 degrees
C, which precipitates 4 nm diameter hematite crystals with the elongated
one-dimensional form. Last, the membrane is soaked in a HCl solution, which
leaches out the hematite. A porous membrane with one-dimensional pores 4 nm in
diameter remains.
The resulting mesoporous films are stable when heated for 2 hours at 600 degrees
C. Isothermal nitrogen gas sorption and desorption show pore surface areas are
more than 1000 sq m/g.
This three-step process can be applied to many oxide eutectic systems for
fabrication of various mesoporous films with one-dimensional through channels,
including films of TiO2, SnO2 and ZrO2.
The technology is patent pending in Japan and US. This research is carried out
and funded by the Japanese government, and currently is not available for
partnerships or joint ventures. | 
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