Density functional theory study on the influence of cation ratio on the host layer structure of Zn/Al double hydroxides

A density functional theory (DFT) study has been carried out for [Znn−1Al(OH2)n+6(OH)2n−2]3+ (n = 3-6) and [Znn−1Al(OH2)2n−2(OH)2n−2]3+ (n = 7) clusters, which include the basic structural information of the brucite-like lattice structure of Zn/Al layered double hydroxides (LDHs) with Zn/Al molar ratio (R) in the range 2-6, in order to understand the effect of the Zn/Al ratio on the structure and stability of binary Zn/Al LDHs. Based on systematic calculations of the geometric parameters and formation energies of the cluster models, it was found that it is possible for Zn2+ and Al3+ cations to replace Mg2+ isomorphously in the brucite-like structure with different R values, resulting in differences in microstructure of the clusters and unit cell parameter a of the Zn/Al LDHs. Analysis of the geometry and bonding around the trivalent Al3+ or divalent Zn2+ cations reveals that Al3+ plays a more significant role than Zn2+ in determining the microstructure properties, formation and bonding stability of the corresponding ZnRAl clusters when R < 5, while the influence of Zn2+ becomes the dominant factor in the case of R ≥ 5. These findings are in good agreement with experiments. This work provides a detailed electronic-level understanding of how the composition of cations affects the microstructure and stability of Zn-containing binary LDH layers.