Preparations of self-supporting nanofilms of metal oxides by casting processes

We recently developed a solution-based method for the preparation of self-supporting (free-standing) titania films with thicknesses less than a micrometre. In this study, we demonstrated its wide applicability to other metal oxides. Film fabrication by spin casting was employed for preparation of self-supporting nanofilms (< 100 nm thickness) from various combinations of metal oxides, polymer underlayers, and polymer middle layers. All the metal alkoxides we examined gave self-supporting ultrathin films of Al2O3, Nb2O5, ZrO2, SiO2 and La2O3. It was found that the photoresist polymer underlayer employed for ready film detachment could be replaced by other commercial polymers. The polymer underlayer could be omitted when a cast acrylic sheet was used as the solid substrate. The importance of the PVA middle layer was confirmed by successful formation of uniform large metal oxide films compared to the case of other polymers. This is attributed to a high population of the surface hydroxyl group that facilitates efficient development of lateral metal oxide network within the film. Admixing of organic carboxylic acids in the casting solution provided similarly uniform metal oxide nanofilms. These results revealed that the spin casting method is applicable to a wide variety of metal oxides and polymer layers. We also developed the flow casting method for film fabrication and its usefulness was discussed. These newer approaches were compared with the surface sol-gel process which we previously described. Among the three casting methods, the spin casting method was most effective for obtaining uniform large nanofilms of metal oxides by simple operation, although it was limited to flat surfaces.