Metal-organic frameworks (MOFs) containing d0 metals such as NH2-MIL-125(Ti), NH2-UiO-66(Zr) and NH2-UiO-66(Hf) are among the most studied MOFs for photocatalytic applications. Despite structural similarities, we demonstrate that the electronic properties of these MOFs are markedly different. As revealed by quantum chemistry, EPR measurements and transient absorption spectroscopy, the highest occupied and lowest unoccupied orbitals of NH2-MIL-125(Ti) promote a long lived ligand-to-metal charge transfer upon photoexcitation, making this material suitable for photocatalytic applications. In contrast, in case of UiO materials, the d-orbitals of Zr and Hf, are too low in binding energy and thus cannot overlap with the π* orbital of the ligand, making both frontier orbitals localized at the organic linker. This electronic reconfiguration results in short exciton lifetimes and diminishes photocatalytic performance. These results highlight the importance of orbital contributions at the band edges and delineate future directions in the development of photo-active hybrid solids.
Hybrid
organic–inorganic perovskites represent a special
class of metal–organic framework where a molecular cation is
encased in an anionic cage. The molecule–cage interaction influences
phase stability, phase transformations, and the molecular dynamics.
We examine the hydrogen bonding in four AmBX3 formate perovskites:
[Am]Zn(HCOO)3, with Am+ = hydrazinium (NH2NH3+), guanidinium (C(NH2)3+), dimethylammonium (CH3)2NH2+, and azetidinium (CH2)3NH2+. We develop a scheme to quantify
the strength of hydrogen bonding in these systems from first-principles,
which separates the electrostatic interactions between the amine (Am+) and the BX3– cage. The hydrogen-bonding
strengths of formate perovskites range from 0.36 to 1.40 eV/cation
(8–32 kcalmol–1). Complementary solid-state
nuclear magnetic resonance spectroscopy confirms that strong hydrogen
bonding hinders cation mobility. Application of the procedure to hybrid
lead halide perovskites (X = Cl, Br, I, Am+ = CH3NH3+, CH(NH2)2+) shows that these compounds have significantly weaker hydrogen-bonding
energies of 0.09 to 0.27 eV/cation (2–6 kcalmol–1), correlating with lower order–disorder transition temperatures.
The production of hydrogen at a large scale by the environmentally-friendly electrolysis process is currently hampered by the slow kinetics of the oxygen evolution reaction (OER). We report a solid electrocatalyst α-Li 2 IrO 3 which upon oxidation/delithiation chemically reacts with water to form a hydrated birnessite phase, the OER activity of which is five times greater than its non-reacted counterpart. This reaction enlists a bulk redox process during which hydrated potassium ions from the alkaline electrolyte are inserted into the structure while water is oxidized and oxygen evolved. This singular charge balance process for which the electrocatalyst is solid but the reaction is homogeneous in nature allows stabilizing the surface of the catalyst while ensuring stable OER performances, thus breaking the activity/ stability tradeoff normally encountered for OER catalysts.
We
report an investigation of the “missing-linker phenomenon”
in the Zr-based metal–organic framework UiO-66 using atomistic
force field and quantum chemical methods. For a vacant benzene dicarboxylate
ligand, the lowest energy charge-capping mechanism involves acetic
acid or Cl–/H2O. The calculated defect
free energy of formation is remarkably low, consistent with the high
defect concentrations reported experimentally. A dynamic structural
instability is identified for certain higher defect concentrations.
In addition to the changes in material properties upon defect formation,
we assess the formation of molecular aggregates, which provide an
additional driving force for ligand loss. These results are expected
to be of relevance to a wide range of metal–organic frameworks.
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