Density Functional Theory Study On The Microscopic Structure Of Layered Double Hydroxides Material

Layered double hydroxides (LDHs), also known as hydrotalcite or anionic clays, have been attracting significant attention from both academia and industry. Because LDHs'layer-sheets and interlayer anions are flexibly controllable, and their structure and properties are changeable, it is widely used in anion exchange, adsorption, catalysis, medicine, and electrochemistry.In this thesis, we present a density functional theory study on the distorted structure and stability of Copper-Zinc-Aluminum LDHs: We propose a periodic interaction model for the LDHs, CuxZn₃-xAl-LDHs(X=0-3). Jahn–Teller effect and the stability are investigated by analyzing the geometric parameters, hydrogen-bonding, charge populations, and binding energies. The results show that the symmetry of unit cell becomes weaker when the amount of Cu²⁺ substituted Zn2 + in the layer increasing, and Jahn–Teller effect of the copper has small effect on the central Al3 +. The strength of electrovalent bond becomes stronger while the strength of covalent bond becomes weaker with the increase of Cu²⁺ substituted Zn2 + in the layer, and the covalent crystal of CuxZn₃-xAl-LDHs is transited to the ionic crystal gradually. The distortion caused by Jahn–Teller effect makes an enhanced super-molecular interaction between the host layer and the guest, the strength of electrostatic interactions become weaker while the strength of hydrogen-bonding becomes stronger and hydrogen-bonding is superior to the electrostatic interaction. In general, the absolute value of the binding energy decreases progressively with the increase of the number of Cu²⁺, which decreases the chemical stability of the system, and it is significant for synthesis of Cu-LDHs.We also provide a theory study on the distorted microstructure of Copper-Zinc-Magnesium-Aluminum layered double hydroxides: Microscopic structure and electronic property of (M)-IV-LDHs(M=Zn, Mg, Cu) are investigated in this paper. The distortion and stability of (M)-IV-LDHs are studies in order to find the best arrangement position for Cu. The results indicate that Jahn-Teller effect does not only exist in the unsaturated d orbit of Cu²⁺ and Zn²⁺, but also in the unsaturated p orbit of Mg²⁺. Moreover, the unsaturated d/p orbital affects the Jahn-Teller distortion of the metal ion together. From (Zn)-IV-LDHs, (Mg)-IV-LDHs to (Cu)-IV-LDHs, it gets more Jahn-Teller stabilization energy from the increase of the average distortion angle of metal, i.e.,ΔθOMO. At the meanwhile, Cu-O octahedral changes from squashed octahedral to elongated octahedral. In general, the absolute value of the binding energy decreases gradually, and the chemical stability of the system gets better, which indicates that the system of (Cu)-IV-LDHs with Cu in the vertex position in the lattice is the ideal structure.Moreover, we study the super-molecular interaction between host layer and guest CO₃^2-, H2O of Magnesium-Tin layered double hydroxides: We investigate the distribution of hydrogen bond, electronic properties and super-molecular interaction between the guest anions, using a simple simulation that inserts H2O into the host layers with different numbers of H2O. When CO₃^2- and H2O are inserted into the layers of Mg3Sn-LDHs, there is a strong super-molecular interaction between the host layers and the guests, including the hydrogen-bonding and electrostatic interactions. Hydrogen-bonding is superior to the electrostatic interaction in the hydration process. In Mg3Sn-LDHs-yH2O (y=0-3), the interlayer distance increases first, and then decreases slightly with the increasing number of interlayer water molecules. When y is equal to 0 or 1, the plane with CO₃^2- and water is parallel to the host layer, and is approximately in the middle of the two layers. When y is equal to 2 or 3, the guest layers are not parallel to the host layer and their distribution is random. The absolute value for the hydration energy decreases gradually with an increase in the number of water molecules, which indicates that the hydration of Mg₃Sn-LDHs has reached a definite saturation state.