Hierarchically Porous Metal Oxide Monoliths Prepared by the Nanocasting Route

In this paper, successful preparations of hierarchically porous cobalt oxide (Co3O4), tin oxide (SnO2), and manganese oxide (MnO2 or Mn2O3) monoliths by the nanocasting route are described. The starting SiO2 monoliths used as molds were prepared through a straightforward sol−gel process and contain macropores with adjustable size in the range of 0.5−30 μm as well as mesopores which can be altered between 3 and 30 nm. In the nanocasting process, the silica monoliths are impregnated with a metal salt solution, which is subsequently decomposed to a metal oxide by heat treatments to form a SiO2/MeOx composite. Finally, the silica part can be removed by leaching in either NaOH or hydrofluoric acid. The composite and replica structures have been characterized by thermogravimetric analysis, X-ray diffraction, scanning electron microscopy, nitrogen physisorption, and transmission electron microscopy. The nanocast monoliths are positive replicas of the silica structure on the micrometer scale, meaning that the replicas have the same macroscopic morphology and macropore structure as the starting silica monoliths. In contrast, on the nanometer scale the replicated structure becomes an inverse (or a negative replica) of the silica mesopore structure. Furthermore, all prepared metal oxide monoliths are fully crystalline. When the hierarchical structure of the monoliths is combined with the unique chemical or physical properties of the used metal oxides, these novel materials have great potential in application areas such as catalysis, HPLC, and sensor materials