Structural and Electronic Properties of Iron-Doped Sodium Montmorillonite Clays: A First-Principles DFT Study

Abstract

First-principles calculations done via density functional theory were used to study the structural and electronic properties of sodium montmorillonite clay (Mt-Na⁺) of general formula M <i>_x</i> Al₃Si₈O₂₄H₄Na·<i>n</i>H₂O (M <i>_x</i> : Mg or Fe). The final position of the interlamellar sodium atom is found to be close to the oxygen atoms located on the upper surface of silica. Following Fe-Mt-Na⁺ system relaxation, with subsequent analysis of magnetic moment and magnetic states, the electroneutrality of the system established that both Fe²⁺ and Fe³⁺ oxidation states are possible to occur. The Mg²⁺-Mt-Na⁺ material shows a band gap energy greater than that of Fe²⁺-Mt-Na⁺ when iron is in the octahedral site. It is found that the valence-band maximum and the conduction-band minimum of iron-doped montmorillonite are both at the Γ-point, while it is at <i>V</i> → Γ for magnesium-doped montmorillonite. The calculated band gap from hybrid functional (HSE06) of Fe²⁺-Mt-Na⁺ is equal to 4.3 eV, exhibiting good agreement with experimental results obtained from ultraviolet-visible spectroscopy of the natural Mt-Na⁺ (Cloisite-Na⁺).