The Structure And Energies Of Mg(Zn)Al-LDHs In Different Interlayer Surroundings:a Density Functional Theory Study

Layered double hydroxides(LDHs)are a class of anionic clays with host-guest structure.They are good templets for the functional layered materials due to the exchanging properties of metal ion and interlayer anions.It is well known that the properties of materials depend on their structure,so the detailed study on the structure of LDHs is quite necessary.In this thesis,the influence of guests and hosts on the structure and properties of LDHs are systematically investigated by density functional theory(DFT)method.The main research contents and conclusions are as follows:(1)The orientation of the interlayer species,interlayer distances,binding energies,electronic density difference and density of states of the MgnAl-LDHs(n=1.6,2.0,2.6,3.5,5.0,and 8.0)with nine different anions(F-,Cl-,Br-,I-,OH-,No3-,CO32-,SO42-,PO43-)are calculated.The results reveal that for the anions with similar size,the lower charge of the anion will lead to the more tilted orientation.The LDHs containing the anions with more inclined arrangement,larger size,lower charge(larger numbers)and with larger number of interlayer water molecules show larger interlayer distance.The higher anion charge leads to larger binding energy for LDHs,the binding energy varies in the order which is the same as the sequence of anion exchange PO43->CO32->SO42->OH->F->Cl->Br->NO3->I-.The interaction of the layer and the interlayer species not only comes from the host-guest electrostatic attraction,but also from the hydrogen bond between hydroxyl H of layer and halogen or O of anions.The interaction between interlayer anions and water molecules also shows the hydrogen bond.The main component of valence band maximum(VBM)and conduction band minimum(CBM)of MgAl-LDHs are derived from p orbital of halogen anions or the O-2p orbital of other anions,and the orbitals of Mg-2p orbital,respectively.And the band gap is predominantly influenced by the metal cations in the LDHs matrix.(2)The structure,unit cell parameters,Mulliken charge and density of states of MgAl-Cl-LDHs and ZnAl-Cl-LDHs were studied.lt was found thatthe guest anions Cl-arrange in the middle area between the two layers and the arrangement is not affected by the metal of the layer.The unit cell parameter value a of the LDHs isproportional to the size of the metal ions in hydroxides layer.Cl-in the MgAl-Cl-LDHs system has a large charge value,indicating that Cl-has more charge exchange.The hydroxyl group and the interlayer anion Cl-are the most alkaline sites in the MgAl-Cl-LDHs system and the ZnAl-Cl-LDHs system,respectively.(3)The structure of Mg2Al-Cl-LDHs with different n values(the multiple of interlayer space),energy and charge difference were studied.The results show that n=1.9-2.3 is thelimitation distance multiple of interlayer space of Mg2Al-Cl-LDHs,beyond this distance.The interaction between the host and the guest of LDHs will become very weak when n>1.9.This work provides effective theoretical information for understanding the guest and host effect on the properties of LDHs,which is helpful for the design and preparation of the functional LDHs or related materials with prospective applications.