Crystallization of β-MnO2 Nanowires in the Pores of SBA-15 Silicas: In Situ Investigation Using Synchrotron Radiation

An original preparation method, called "two solvents" method, allows the production of MnO2 nanowires patterned by SBA-15 silicas under mild conditions, with a preserved two-dimensional hexagonal structure, a 97% filling of the porosity by oxide nanowires, and a controlled microstructure. A comparison is made with Mn-loaded SBA-15 prepared by more conventional adsorption methods. In the latter case, MnOx particles inside and outside the silica grains, empty and filled mesopores, and several Mn oxides (MnO2, Mn2O3, and Mn3O4) were identified. Once the preparation method of Mn-loaded SBA-15 optimized, various X-ray scattering and adsorption techniques using synchrotron radiation were used to observe salient features of the MnO2 nanowires crystallization in situ upon calcination. X-ray absorption at the Mn K edge shows that the oxidation state of manganese increases from (II) to (IV) between 80 and 120 °C. The oxidation of the Mn(II) salt occurs at a temperature lower than that necessary for bulk manganese nitrate (200 °C), which confirms its confinement within the SBA-15 pores. β-MnO2 nanowires of defective pyrolusite type are identified by wide-angle diffraction. The comparison between diffraction results and simulations demonstrates that the nanowire diameter is similar to the mesopore diameter of the silica host. A small contraction of unit-cell parameters occurs upon the crystallization of β-MnO2 nanowires. A parallel overall intensity increase observed in small-angle X-ray diffraction is the fingerprint of a homogeneously filled porosity.