Solid state microwave synthesis of highly crystalline ordered mesoporous hausmannite Mn₃O₄ films

Soft templating using block copolymers provides a generalized synthetic strategy to fabricate mesoporous materials, but it is generally challenging to obtain highly crystalline frameworks without significant deformation or loss of the mesoporous structure. Here, we demonstrate a simple route to generate ordered mesoporous crystalline manganese oxide films by block copolymer templating through microwave processing to convert carbonate precursors to oxides, remove the polymeric template and crystallize the Mn₃O₄, all within 1 min. The microwave heating in this case is driven primarily by the high microwave cross-section of the substrate (silicon wafer), but manganese oxide also absorbs microwaves to provide energy locally for promoting nucleation/crystallization. Conversely, conventional heating in a muffle furnace at an analogous surface temperature leads to either significant residual copolymer or nanostructure collapse with low crystallinity. This difference in the behavior is attributed to the rapid and local heating of the manganese oxide by microwaves to crystallize the oxide. Microwaves rapidly generate the crystals as evidenced by the invariance in the refractive index of the films after 45 s on further microwave heating. Additionally, the microwave processing leads to nearly twice the specific surface area for the films than that of mesoporous films fabricated by calcination in the furnace. Microwave energy appears to be an attractive alternative to enable the fabrication of a highly crystalline framework in soft-templated ordered mesoporous materials when the microwaves can be absorbed by the framework of interest.