Formation of manganese oxide shells on silica spheres with various crystal structures using surfactants for the degradation of methylene blue dye

In this research, silica–Mn oxide nanocomposites with a stable core–shell structure were prepared successfully by adding a Mn aqueous solution containing Mn nitrate, CTAB, and t-butanol into the colloid of spherical silica particles under basic condition. Mn oxide shells with a range of thickness from 7.92 nm to 29.89 nm could be formed on the surface of silica nanoparticles by changing the concentrations of TEOS and Mn nitrate as the precursors of silica and Mn oxide. As the higher concentration of Mn nitrate was added into the colloid of smaller silica particles, the Mn oxide shell became thicker. In addition, the crystal structure and the surface characteristics of silica–Mn oxide nanocomposites could be controlled efficiently by varying the calcination temperature. When the products were calcined at 500 °C and 800 °C, the pure crystal phase and the improved crystallinity of Mn3O4 and Mn2O3 in the composites could be obtained, respectively. Through this synthetic process, the thickness of Mn oxide shell could be increased up to 34.40 nm. The formed silica–Mn oxide composite particles were characterized by using FE-SEM, TEM, XRD, EDS, and BET. The catalytic performances of composites for the degradation of methylene blue (MB) in water were investigated in the presence of H2O2 at 25 °C. The silica–Mn3O4 composite particles synthesized through calcination at 500 °C for 6 h showed the highest catalytic activity for the degradation of MB. The degradation efficiencies of composites for MB were analyzed by using UV–vis spectrophotometer.

Graphical abstractCatalytic activity of the synthesized silica–Mn oxide nanocomposites for the degradation of MB in the presence of H2O2: (a) pure MB, (b) silica–(Mn2O3 + Mn3O4) (calcined at 700 °C for 6 h), (c) silica–Mn3O4 (not calcined), (d) silica–Mn2O3 (calcined at 800 °C for 6 h), and (e) silica–Mn3O4 (calcined at 500 °C for 6 h).Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Silica–Mn oxides with core–shell structure were formed using surfactant solution. ► Thickness of Mn oxide on silica was increased as Mn salt amount was increased. ► MB degradation by the prepared composites was enhanced greatly with the aid of H2O2.