Periodic DFT+D Molecular Modeling of the Zn-MOF-5(100)/(110)TiO2 Interface: Electronic Structure, Chemical Bonding, Adhesion, and Strain
نویسندگان
چکیده
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 [11 ̅0] 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[11 ̅0] = 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/nm) stems from the sizable dispersion (−0.39 eV/nm) 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. ■ INTRODUCTION Owing to their unique structure and the related properties such as designable topology, tunable metrics, and versatile functionalities, metal−organic frameworks (MOF) belong to the well-establish class of advanced hybrid materials of broad scientific interest. The MOF structure is based on a combination of two essential building components: an inorganic part (constituted by metal or metal−oxo clusters) and a functional organic part acting as a linker. Their ultrahigh apparent surface areas and inner porosities (up to 90% free volume), together with extraordinary chemical changeability of both the inorganic building units and the organic connectors, give rise to a wide range of spectacular applications. Indeed, they are used as gas storage and molecular sieving materials, catalysts for fine chemicals, drug delivery systems, or chemical sensors. Various synthetic strategies have been developed for designing and manufacturing a vast range of good-quality bulk MOF materials, and they have been reviewed several times elsewhere. Typically, MOF materials are prepared in the form of powders by conventional chemical methods, yet the resultant microcrystals are often not of the desired quality. Their size and shape are usually imaged by using scanning electron microscopy, and several interrelated transmission electron microscopy techniques such as high resolution transmission electron microscopy (HR-TEM), HAADFSTEM, or electron diffraction. The first successful ultrahigh resolution imaging of MOF-5 crystals using aberration-corrected cryo-TEM (80 keV) has been published recently by Wiktor et al., opening a new field for direct structure observations of those fragile materials with atomic resolution. For a number of important applications (e.g., functionalized membranes and sensors, solar materials, electronic and light emitting devices), homogeneous, compact, dense, welloriented, and defect-free MOF thin films (surface-attached metal−organic framework layers known as SURMOFs) are of special interest. However, despite extensive studies in this field only a few reports on the deposition of MOF on solid substrates are available at present. Following Fischer, two classes of MOF films can be distinguished: polycrystalline films and thin surface MOF layers. Polycrystalline films may be regarded as randomly oriented MOF crystallites covering the surface of a substrate in a compact or scarce fashion. Their thickness may arrive at a micrometric scale, often mere confinement rather than substrate related effects are responsible for orienting the growth. However, chemical interactions with the surface may Received: December 30, 2013 Revised: March 17, 2014 Article
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Chemical bonding at the metal–organic framework/metal oxide interface: simulated epitaxial growth of MOF-5 on rutile TiO2
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