Theoretical Study on the Swelling Mechanism and Structural Stability of Ni3Al-LDH Based on Molecular Dynamics.

layered double hydroxide (LDH) as a kind of 2D layer material has a swelling phenomenon. Because swelling significantly affects the adsorption, catalysis, energy storage, and other application properties of LDHs, it is essential to study the interlayer spacing, structural stability, and ion diffusion after swelling. In this paper, a periodic computational model of Ni3Al-LDH is constructed, and the supramolecular structure, swelling law, stability, and anion diffusion properties of Ni3Al-LDH are investigated by molecular dynamics theory calculations. The results show that the interlayer water molecules of Ni3Al-LDH present a regular layered arrangement, combining with the interlayer anions by hydrogen bonds. As the number of water molecules increases, the hydrogen bond between the anion and the basal layer gradually weakens and disappears when the number of water molecules exceeds 32. The hydrogen bond between the anion and the water molecule gradually increases, reaching an extreme value when the number of water molecules is 16. The interlayer spacing of Ni3Al-LDH is not linear with the number of water molecules. The interlayer spacing increases slowly when the number of water molecules is more than 24. The maximum layer spacing is stable at around 19 Å. The interlayer spacing, binding energy, and hydration energy show an upper limit for swelling: the number of water molecules is 32. When the number of interlayer water molecules is 16, the water molecules' layer structure and LDH interlayer spacing are suitable for anions to obtain the maximum diffusion rate, 10.97 × 10-8 cm2·s-1.

First-Principles Study on Interlayer Spacing and Structure Stability of NiAl-Layered Double Hydroxides.

Interlayer spacing and structure stability of layered double hydroxides (LDHs) on their application performance in adsorption, ion exchange, catalysis, carrier, and energy storage is important. The effect of different interlayer anions on the interlayer spacing and structure stability of LDHs has been less studied, but it is of great significance. Therefore, based on density functional theory (DFT), the computational model with 10 kinds of anions intercalated Ni3Al-A-LDHs (A = Cl-, Br-, I-, OH-, NO3 -, CO3 2-, SO4 2-, HCOO-, C6H5SO3 -, C12H25SO3 -) and four Ni R Al-Cl-LDH models with different Ni2+/Al3+ ratios (R = 2, 3, 5, 8) were constructed to calculate and analyze interlayer spacing, structural stability, and their influence factors. It was found that the interlayer spacing order of Ni3Al-A-LDHs intercalated with different anions is OH- < CO3 2- < Cl- < Br- < I- < HCOO- < SO4 2- < NO3 - < C6H5SO3 - < C12H25SO3 -. The hydrogen bond network between the base layer and the interlayer anions affects the arrangement structure of the interlayer anions, which affects the interlayer spacing. For interlayer monatomic anions Cl-, Br-, and I- and the anion of comparable size in each direction SO4 2-, the interlayer spacing is positively correlated with the interlayer anion diameter. The larger difference between the long-axis and short-axis dimensions of the polyatomic anions results in the long axis of the anion being perpendicular to the basal layer, increasing interlayer spacing. The long-chain anion C12H25SO3 - intercalation system exhibits the largest layer spacing of 24.262 Å. As R value increases from 2 to 8, the interlayer spacing of Ni R Al-Cl-LDHs gradually increases from 7.964 to 8.124 Å. The binding energy order between the interlayer anion and basal layer is CO3 2- > SO4 2- > OH- > Cl- > Br- > I- > HCOO- > NO3 - > C12H25SO3 - > C6H5SO3 -. The smaller the interlayer spacing, the higher the binding energy and the stronger the structural stability of LDHs. The factors affecting structural stability mainly include the bond length and bond angle of the hydrogen bond and the charge interaction between the basal layer and interlayer anion. In the CO3 2- intercalated system, the hydrogen bond length exhibits the shortest of 1.95 Å and the largest bond angle of 163.68°. The density of states and energy band analysis show that the higher the number of charges carried by the anion, the stronger its ability to provide electrons to the basal layer.