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Register a new userConnecting Defects and Amorphization in UiO-66 and MIL-140 Metal-organic Frameworks: A Combined Experimental and Computational Study
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The mechanism and products of the structural collapse of the metal-organic frameworks (MOFs) UiO-66, MIL-140B and MIL-140C upon ball-milling are investigated through solid state 13C NMR and pair distribution function (PDF) studies, finding amorphization to proceed by the breaking of a fraction of metal-ligand bonding in each case. The amorphous products contain inorganic-organic bonding motifs reminiscent of the crystalline phases. Whilst the inorganic Zr6O4(OH)4 clusters of UiO-66 remain intact upon structural collapse, the ZrO backbone of the MIL-140 frameworks undergoes substantial distortion. Density functional theory calculations have been performed to investigate defective models of MIL-140B and show, through comparison of calculated and experimental 13C NMR spectra, that amorphization and defects in the materials are linked.
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The metal-organic framework (MOF) MFU-4l containing Co(II) centers and Cl- ligands has recently shown promising redox activity. Aiming for further improved MOF catalysts for oxidation processes employing molecular oxygen we present a density-functional theory (DFT) based computational screening approach to identify promising metal center and ligand combinations within the MFU-4l structural family. Using the O2 binding energy as a descriptor for the redox property, we show that relative energetic trends in this descriptor can reliably be obtained at the hybrid functional DFT level and using small cluster (scorpionate-type complex) models. Within this efficient computational protocol we screen a range of metal center / ligand combinations and identify several candidate systems that offer more exothermic O2 binding than the original Co/Cl-based MFU-4l framework.
Thermally-densified hafnium terephthalate UiO-66(Hf) is shown to exhibit the strongest isotropic negative thermal expansion (NTE) effect yet reported for a metal-organic framework (MOF). Incorporation of correlated vacancy defects within the framework affects both the extent of thermal densification and the magnitude of NTE observed in the densified product. We thus demonstrate that defect inclusion can be used to tune systematically the physical behaviour of a MOF.
Metal-Organic Frameworks (MOFs) have gained much interest due to their intrinsic tunable nature. In this work, we study how linker functionalization modifies the electronic structure of the host MOF, more specifically the MIL-47(V)-R (R=-F, -Cl, -Br, -OH, -CH$_3$, -CF$_3$, and -OCH$_3$). It is shown that the presence of a functional group leads to a splitting of the $pi$-orbital on the linker. Moreover, the upward shift of the split-off $pi$-band correlates well with the electron withdrawing/donating nature of the functional groups. For halide functional groups the presence of lone-pair back donation is corroborated by calculated Hirshfeld-I charges. In case of the ferromagnetic configuration of the host MIL-47(V$^{mathrm{+IV}}$) material a half-metal to insulator transition is noted for the -Br, -OCH$_3$, and -OH functional groups, while for the anti-ferromagnetic configuration only the hydroxy-group results in an effective reduction of the band gap.
The defect chemistry of perovskite compounds is directly related to the stoichiometry and to the valence states of the transition metal ions. Such relations are of high interest as they offer the possibility to influence the catalytic activity of perovskites for the application in solid-oxide fuel- and electrolyser cells. Combining theoretical and experimental approaches, we explore the feasibility of actively manipulating the valence state of Fe and the concentration of point defects by synthesizing non-stoichiometric LaFeO$_3$ (LFO). In the theoretical part, formation energies and concentrations of point defects were determined as a function of processing conditions by first-principles DFT+U calculations. Based on the DFT+U results, significant compositional deviations from stoichiometric LFO cannot be expected by providing rich or poor conditions of the oxidic precursor compounds (Fe$_2$O$_3$ and La$_2$O$_3$) in a solid-state processing route. In the experimental part, LFO was synthesized with a targeted La-site deficiency. We analyze the resulting phases in detail by X-ray diffraction and dedicated microscopy methods, namely scanning electron microscopy (SEM) and (scanning) transmission electron Microscopy ((S)TEM) in combination with energy dispersive X-ray spectroscopy (EDS) and electron energy-loss spectrometry (EELS). Instead of a variation of the La/Fe ratio, a mixture of two phases, Fe$_2$O$_3$/LaFeO$_3$, was observed resulting in an invariant charge state of Fe, which is in line with the theoretical results. We discuss our findings with respect to partly differing assumptions made in previously published studies on this material system.
We study the oxo-hexametallate Li$_7$TaO$_6$ with first-principles and classical molecular dynamics simulations, obtaining a low activation barrier for diffusion of $sim$0.29 eV and a high ionic conductivity of $5.7 times 10^{-4}$ S cm$^{-1}$ at room temperature (300 K). We find evidence for a wide electrochemical stability window from both calculations and experiments, suggesting its viable use as a solid-state electrolyte in next-generation solid-state Li-ion batteries. To assess its applicability in an electrochemical energy storage system, we performed electrochemical impedance spectroscopy measurements on multicrystalline pellets, finding substantial ionic conductivity, if below the values predicted from simulation. We further elucidate the relationship between synthesis conditions and the observed ionic conductivity using X-ray diffraction, inductively coupled plasma optical emission spectrometry, and X-ray photoelectron spectroscopy, and study the effects of Zr and Mo doping.
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