Theoretical Study On The Construction Of Layered Double Hydroxides Structure

Layered double hydroxides(LDHs) have received considerable attention as a good template for the design and assembly of functional host-guest materials.However,some generally accepted empirical rules for the LDHs synthesis can not give rational explanation for many experimental findings.Since the close relationship between the material properties and structures has been confirmed thus far,the theoretical studies in this dissertation were carried out in order to understand the microstructure of LDHs at the scale of atomic and electronic,explain the unclear experimental facts,and find out more rational rules for the design and preparation of LDHs materials.The molecular cluster and periodic solid models have been used for the calculation of the electronic property of metal cations in LDHs layer, the composition,ordering and stacking pattern of layers as well as the property of interlayer anions.The geometry,binding,formation and stabilization energy,natural bond orbital and vibration property of the cluster model were computed by density functional theory(DFT) method. In the case of the periodic model,the calculations were performed using the plane-wave pseudopotential implementation of DFT.The initial template unit of LDHs layers,MO₆([M(OH₂)₆]ⁿ⁺(M= metal cation,n=1,2,3,4),has been investigated.A distortion-angle rule was proposed that the metal cations with the distortion angle(θ) smaller than 1°are easily incorporated into LDHs layers to form stable structures, while those withθlarger than 10°are difficult to be introduced into LDHs layers.Some of the experimental findings which can not be explained by the empirical ion-size rule can be well understood by the calculation-based rule.A database of the calculated geometric parameters has been thus established.The[M₂Al(OH₂)₉(OH)₄]³⁺ clusters(M=divalent cation Mg²⁺,Ca²⁺, Mn²⁺,Fe²⁺,Co²⁺,Ni²⁺,Cu²⁺,Zn²⁺ or Cd²⁺) have been studied.The calculated binding energies are in good agreement with the relative stability of the experimental results for the corresponding LDHs.The 2Ni-Al cluster shows the highest stability among the open-shelled-cation-containing clusters,while the stability of the 2Cu-Al cluster is the weakest.The 2Mg-Al and 2Zn-Al cluster are the most stable ones among the closed-shelled-cation-containing clusters.The electronic structure of divalent cations plays a more significant role in the structure and stability of the corresponding LDHs layers rather than ionic size.The[Mg_n-1M(OH₂)_n+6(OH)_2n-2]³⁺(n=3～6) and[Mg_n-1M(OH₂)_2n-2 (OH)_2n-2]³⁺(n=7)(M=Al,Ga) clusters have been investigated.The stability of both MgAl and MgGa cluster increases with the decrease of M²⁺/M³⁺ ratio(R).The trivalent cation(Al³⁺ or Ga³⁺) plays a more significant role than Mg²⁺ in the microstructure and bonding properties of the corresponding Mg_n-M(â…¢)(M=Al,Ga) clusters,especially when R<5; while Mg²⁺ becomes a dominant factor when R≥5.These findings imply the R range(R=2～5) for the stable formation of pure LDH phase.The calculation of disordered unit cell of Mg-Al-Cl-LDH shows that the probable R(Mg/Al ratio) range for the stable formation of pure LDH phase is R=2～4.The stability of the Mg-Al-Cl-LDH unit cell increases upon increasing R due to the Al-O-Al bond therein.The Al content decreases with the increase of R,which leads to the decrease of electron density of 3s and 3p in metal cation and that of 2p in Cl.Consequently, the HOMO-LUMO energy gap as well as the systematical stability increase.From this point of view,the Lowenstein rule was substantiated that the Al-O-Al bond is absent in LDHs layers.The disordered unit cells of Mg-Al-Cl-LDH with three different stacking sequence(3R,2H,1H) were calculated respectively.The results reveal that system with 3R polytype is the most stable one with the same Mg/Al ratio.This finding is related to the reason that the conductive band energy levels decline with the increase of electron density of 3s,3p of metal cation in 3R system. The 3R Mg-Al-Cl-LDH is more stable than that of Mg-Al-LDH-layer due to the intercalation of Cl anion.This stability also leads to the significant decrease of unit cell parameters,bond angles, bond angles and basal spaces in Mg-Al-Cl-LDH,along with the increase of the layer thickness.These changes result from the superposition of the electron density of 2p in Cl and that in O,which increases the dislocation of 2p electrons as well as the interaction between host layers and interlayer anions.The theoretical calculations in this dissertation not only provided a number of theoretic information of LDHs structure,but also clarified some unclear experimental facts.This work therefore will play an important role to instruct the design and preparation of LDHs or related materials with prospective applications.