Interfacial synthesis: Amphiphilic monomers assisted ultrarefining of mesoporous manganese oxide nanoparticles and the electrochemical implications

Amphiphilic monomers, namely pyrrole and aniline, were used to reduce permanganate ion (MnO₄^-) at the dichloromethane/water interface for the preparation of ultrafine manganese oxide (MnO_x, x ≤ 2) nanoparticles (NPs). These monomers did not undergo polymerization upon oxidation by MnO₄^-, but exerted an interesting effect of ultrarefining the produced MnO_x NPs from reducing MnO ₄^- at the organoaqueous interface. This was attributed to the ability of the monomer to access the interfacial reaction sites from both organic and aqueous phases, and hence retard the as-produced MnO_x nuclei from aggregation at the interface. Such obtained products were mesoporous matrixes of three-dimensionally interconnected and uniform pseudospherical MnO_x NPs (<20 nm). On the contrary, using a more hydrophobic monomer, i.e., o-aminophenol, to reduce MnO₄^- produced a composite of nanobelts of poly(o-aminophenol) embedded in micrometer-sized MnO_x blocks. The ultrafine MnO_x NPs prepared from using aniline or pyrrole exhibited highly capacitive behavior in aqueous Na ₂SO₄, promising their use in supercapacitors. It was also found that the MnO_x NPs prepared from pyrrole-assisted synthesis possessed higher specific capacitance than that from aniline-assisted synthesis, despite the latter having a higher specific surface area. This difference is discussed in terms of crystallographic properties and water contents of these two samples.