Development Of A Force Field For Layered Double Hydroxides And Investigations On Their Topotactic Transformation

Molecular dynamics (MD) simulations play a key role in the design and fabrication of advanced functional layered double hydroxides (LDHs) materials from the viewpoint of structure-property correlation. In this dissertation, a valence force field named LDHFF was developed for the MD study of LDHs. This force field was designed and paramaterized by calculating the structural and electronic properties of octahedral cluster models, which works effectively and accurately for understanding the structural property and exploiting the fabrication of functional LDHs. Furthermore, the topotactic transformation mechanism of LDHs was systematically investigated by a theoretical-experimental combination study, and the involved topological invariant was clearly demonstrated at atomic level. The main results of this dissertation are summarized as follows:(1) Development of a valence force field for LDHsA valence force field (LDHFF), was developed for the MD simulation studies of LDHs. The potential function of LDHFF was specially designed by introducing a double well potential to describe the octahedral structure of host sheets. Each term in the potential function, including the bond stretching constants, angle bending coefficients, cross terms and nonbonded reactions, was parameterized by density function theory (DFT) calculations on the representative cluster models ([M3(OH2)9(OH)4]"+). To validate these potential parameters, MD simulations were subsequently performed on a series of typical LDH models, and the resulting structures, vibrations as well as binding energies were in high agreement with the experimental observations. A 2.0 ns calculation was employed to further validate the stability of LDHFF in long time MD simulations, and it was shown that the metal cations occupy the coordination centers during the whole simulation. These results demonstrate that LDHFF can work effectively and accurately MD simulations of LDH materials, which provides a fundamentally theoretical insight for understanding the structural property of LDHs and related materials.(2) The topotactic transformation mechanism of LDHsThe thermal topotactic transformation mechanism of MgAl-LDHs was studied by MD simulations combied with experimental methods. Thermal analysis (TG-DTA) and in situ XRD results reveal that the LDH phase undergoes three key endothermic events at 330,450 and 800℃, respectively. MD simulations show that the LDH decomposes to CO2 and residual O atoms via a monodentate configuration at 330℃ but nicely preserves its layered structure. At 450℃, it is found that the metal cations almost maintain their original distribution within the LDH(001) facet during the thermal structure-evolution process, but migrate substantially along the c-axis direction perpendicular to the (001) facet. This demonstrates that the metal arrangement/dispersion in LDH matrix is maintained two-dimensionally as the topological invariant during topotactic transformation of LDHs. A complete collapse of layered structure occurs at 800℃, resulting in plenty of holes in the final product. The understanding on structural topotactic transformation process of LDHs at atomic level would give helpful instructions for the design and preparation of nanocatalysts/adsorbents derived from LDHs precursors.