Periodic DFT+D molecular modeling of the $Zn-MOF-5(100)/(110)TiO_{2}$ interface : electronic structure, chemical bonding, adhesion and strain

Electronic structure, bonding characteristics, adhesion, and stress energy of the Zn-MOF-5(100)/(110) rutile interface were modeled by using periodic DFT+D calculations, corroborated by simulation of high resolution transmission electron microscopy (HR-TEM) images. Adjustment of the flexible metal–organic framework (MOF) moiety to the rigid rutile substrate was achieved within a supercell comprised of (1 × 1) Zn-MOF-5 and (4 × 9) TiO2 units. It was shown that binding of the Zn-MOF-5 layer takes place via bidentate 1,4-benzenedicarboxylate (BDC)–titania bridges. A coherent interface can be formed with the minimal periodicity along the [11̅0] direction defined by nine Ti5c adsorption sites (9 × 2.96 Å = 26.64 Å) and two consecutive linkers of the Zn-MOF-5 chain (2 × 12.94 Å = 25.88 Å). The MOF part is tuned to the oxide substrate by tilting the BDC linkers by 10° and twisting around their long axis by 34°. The resultant lattice strain of the Zn-MOF-5 layer was equal to ε[001] = 0.31% and ε[11̅0] = 2.86%, and the associated stress energy to σtotal = 4.8 eV. Pronounced adhesion energy of the Zn-MOF-5 layer deposited on the rutile surface (−0.33 eV/nm2) stems from the sizable dispersion (−0.39 eV/nm2) contribution, counterbalancing the unfavorable lattice strain and bonds distortion components. The calculated density of states structure of the Zn-MOF-5(100)/(110)TiO2 interface revealed that it can be described as an electronically coupled, staggered (Type II) charge injection system, where a photoinduced electron may be directly transferred from the Zn-MOF-5 moiety to the conduction band of the titania substrate