Microscopic mechanism of Fe(Ⅱ/Ⅲ) substitution in lattice on band gap of coal-series kaolinite

To investigate the microscopic mechanism of band gap modulation in coal-bearing kaolinite caused by Fe(Ⅱ/Ⅲ) lattice substitution, coal-bearing kaolinite and Fe-doped kaolinite were selected as research objects. The optical band gaps are measured utilizing ultraviolet-visible diffuse reflectance spectroscopy. Three bulk lattice models are constructed to representdifferent Fe substitution modes: Fe(Ⅱ) substituting Al(Ⅲ) (Fe(Ⅱ)_Al), Fe(Ⅲ) substituting Al(Ⅲ) (Fe(Ⅲ)_Al), and Fe(Ⅲ) substituting Si(IV) (Fe(Ⅲ)_Si). The properties of these Fe(Ⅱ/Ⅲ)-substituted kaolinite unit cells are simulated using Density Functional Theory (DFT) with LDA+U correction to analyze changes in band gap behavior at the atomic level. UV-visible diffuse reflectance spectroscopy results revealed that the optical band gaps of coal-bearing kaolinite and Fe-doped kaolinite were calculated as 3.06 and 2.47 eV, respectively, via the intercept method, and 3.14 and 2.06 eV, respectively, using the Tauc method. Simulations showed thatperfectkaolinite possesses an indirect band gap of 5.055 eV, while Fe-substituted kaolinite models—Fe(Ⅱ)_Al, Fe(Ⅲ)_Al, and Fe(Ⅲ)_Si—exhibited significantly reduced indirect band gaps of 2.121, 0.566, and 0.345 eV, respectively, as calculated using LDA+U correction.These findingssuggest that Fe lattice substitution significantly markedly lowers the band gap of kaolinite,transforming it from an indirect band gap insulator to an indirect band gap semiconductor. The contribution order of Fe 3d orbital electrons near the Fermi level is: Fe(Ⅱ)_Al < Fe(Ⅲ)_Si ≈ Fe(Ⅲ)_Al. The combined analysis of experimental testing and DFT simulations suggests that setting the U value to 2.5 eV and applying Scissors corrections of 0—0.5 eV for Fe(Ⅱ)_Al and 1.5 eV for both Fe(Ⅲ)_Al and Fe(Ⅲ)_Si yielded simulated results that closely align with experimental data.The microscopic mechanism behind the band gap reduction was explained through simulations: ① In perfect kaolinite, the valence band maximum is primarily dominated by O 2p orbitals, while the conduction band minimum is dominated by H 1s and Si 3s orbitals. However, with Fe substituting in the lattice,both the valence band maximum and conduction band minimum are dominated by Fe 3d orbitals, leading to band gap changes. ② Fe substituting in the lattice introduces delocalized 3d states and new hybrid states, occupying the previously wide band gap and significantly reducing the band gap of Fe-substituted kaolinite. ③ Fe(Ⅱ) lattice substitution primarily reduced the band gap by raising the energy of valence-band maximum, a process driven by chemical bond reorganization and electron delocalization.④ Fe(Ⅲ) lattice substitution lowers the conduction band minimum energy due to weakened electron binding forces and the formation of Fe impurity states, which shift the conduction band downward.The combined experimental and DFT results unveiled the microscopic mechanisms through which Fe(Ⅱ/Ⅲ) lattice substitution significantly reduces the band gap of kaolinite. This study provides valuable theoretical insights for optimizing kaolinite's applications in catalysis and functional materials, as well as for guiding doping modulation.