Characterization of the Fe-doped mixed-valent tunnel structure manganese oxide KOMS-2

A sol-gel-assisted combustion method was used to prepare Fe-doped manganese oxide octahedral molecular sieve (Fe-KOMS-2) materials with the cryptomelane structure. Characterization of the nanopowder samples over a wide range of Fe-doping levels (0 ≤ Fe/Mn ≤ 1/2) was carried out using a variety of experimental techniques. For each sample, Cu Kα XRD and ICP-AES were used to index the cryptomelane structure and determine the elemental composition, respectively. A combination of SEM and TEM images revealed that the morphology changes from nanoneedle to nanorod after Fe doping. Furthermore, TGA scans indicated that the thermal stability is also enhanced with the doping. Anomalous XRD demonstrated that the Fe ions replace the Mn ions in the cryptomelane structure, particularly in the (211) planes, and results in a lattice expansion along the c axis, parallel to the tunnels. Reasonable fits to EXAFS data were obtained using a model based on the cryptomelane structure. Mössbauer spectra for selected Fe-KOMS-2 samples indicated that the Fe is present as Fe³⁺ in an octahedral environment similar to Mn in the MnO ₆ building blocks of KOMS-2. Magnetization measurements detected a small amount of γ-Fe₂O₃ second phase (e.g., 0.6 wt % for the Fe/Mn = 1/10 sample), the vast majority of the Fe being in the structure as Fe³⁺ in the high-spin state.