Effects of Fe doping on the structures and properties of hexagonal birnessites - Comparison with Co and Ni doping

Fe-doped hexagonal birnessite was synthesized by adding Fe3+ to the initial reactants, and the effects of Fe doping on the structures and properties of birnessite were investigated and compared with the effects of Co and Ni doping. The underlying mechanisms controlling the incorporation of transition metals (TMs) into the birnessite structure were proposed. Compared to the un-doped control, Fe-doped birnessite has weaker crystallinity, i.e., less stacking of the phyllomanganate sheets in the c direction, and larger surface area. Combination of X-ray photoelectron spectroscopy (XPS) and Mn K-edge XANES and EXAFS spectra demonstrates that Fe doping decreases the Mn average oxidation state (AOS) but has little effect on the basic layer structure and local Mn environments. Fe(III) located in the birnessite layers exhibits high-spin (HS) configuration whereas layer Mn(III) and Co(III) plausibly adopt low-spin (LS) state. The TMs decrease the thickness of birnessite plate crystals along the c axis and affect the unit cell parameter b in the order Fe > Ni > Co. Co and Fe incorporate into the birnessite layers by substitution for Mn(IV) while Ni substitutes for Mn(III). The substitution of TMs into the birnessite layers is governed by the coordination radius (CR), crystal field stabilization energy (CFSE) and oxidation state of the TMs. The variations in potassium contents in doped birnessites together with TM K-edge EXAFS data indicate that most of the Fe (similar to 81-82%) or Ni (similar to 66-76%) incorporated into the birnessite structure exists in the interlayer regions, while most of the Co (similar to 71-80%) occurs in the manganese layers. The compatibility of these TM ions in the birnessite layers is in the order Co > Ni > Fe. The smaller the difference between the CR of Fe, Co or Ni and Mn(IV) or Mn(III), the more dopants are compatible within the Mn layers.