Electronic and vibrational properties of a MOF-5 metal–organic framework: ZnO quantum dot behaviour

Electrochemical capacitance of Ni-doped metal organic framework and reduced graphene oxide composites: more than the sum of its parts

Composites of a Ni-doped metal organic framework (MOF) with reduced graphene oxide (rGO) are synthesized in bulk (gram scale) quantities. The composites are composed of rGO sheets, which avoid restacking from the physical presence of MOF crystals. At larger concentration of rGO, the MOF crystals are distributed on the overlapping and continuous rGO sheets. Ni in Ni-doped MOF is found to engage in a two-electron, reversible, efficient, redox reaction shuttling between Ni and Ni(OH)2 in aqueous potassium hydroxide (KOH) electrolyte. The reaction is rather unique as Ni-based supercapacitors use a one-electron transfer Faradaic redox reaction between Ni(OH)2 and NiO(OH). Employing electrochemical impedance spectroscopy, we determined the charge transfer resistance to be 184 mΩ for MOF, 74 mΩ for a Ni-doped MOF and 6 mΩ for a rGO-Ni-doped MOF composite, but these modifications do not affect the mass transfer resistance. This novel redox reaction in conjunction with the lowered charge transfer resistance from the introduction of rGO underpins the synergy that dramatically increases the capacitance to 758 F/g in the rGO-Ni-doped MOF composite, when the parent MOF could store only 100 F/g and a physical composite of rGO and Ni-doped MOF could algebraically achieve about 240 F/g. A generic approach of doping MOFs with a redox active metal and forming a composite with rGO transforms an electro-inactive MOF to high capacity energy storage material with energy density of 37.8 Wh/kg at a power density of 227 W/kg. These results can promote the development of high-performance energy storage materials from the wide family of MOFs available.

Four new metal–organic frameworks based on bi-, tetra-, penta-, and hexa-nuclear clusters derived from 5-(phenyldiazenyl)isophthalic acid: syntheses, structures and properties

Four new metal–organic frameworks (MOFs) were synthesized under solvothermal conditions based on a new rigid ligand 5-(phenyldiazenyl)isophthalic acid (H2PDIA) with azobenzene.

Multifunctional polyoxometalates encapsulated in MIL-100(Fe): highly efficient photocatalysts for selective transformation under visible light

Multifunctional H₃PMo₁₂O₄₀ encapsulated in MIL-100(Fe) as dual functional visible-light-driven photocatalysts for selective transformation under visible light.

Structural characterization, optical properties and photocatalytic activity of MOF-5 and its hydrolysis products: implications on their excitation mechanism

Hydrolysis of MOF-5 gives place to a mixture of two compounds: MOF-5W and zinc terephthalate dihydrate. The analysis of optical and photochemical properties allows to infer the preeminence of a ligand to ligand charge-transfer.

Ab initio calculation of electronic charge mobility in metal–organic frameworks

The electron mobility of a Zr-UiO-66 benzenedicarboxylate (BDC) metal-organic framework (MOF) with three functional designs was investigated using a DFT method in combination with a Boltzmann relaxation time approximation. The results provide evidence of strong control of the charge carrier mobility in functionalized MOFs through manipulation of the majority carrier population.

Homo- and heterometallic luminescent 2-D stilbene metal-organic frameworks

Metal-organic frameworks (MOFs) can provide a matrix for the assembly of organic chromophores into well-defined geometries, allowing for tuning of the material properties and study of structure-property relationships. Here, we report on the effect of the coordinated metal ion on the luminescence properties of eight isostructural MOFs having the formula M(1)2M(2)L3(DMF)2 (M(1) = M(2) = Zn (1), Cd (2), Mn (3), Co (4); M(1) = Zn, M(2) = Cd (5), Mn (6), Co (7); M(1) = Co, M(2) = Mn (8); L = trans-4,4'-stilbene dicarboxylate), synthesized by reaction of the appropriate metal nitrate or mixtures of metal nitrates with LH2 in DMF. The crystal structures of 2, 3 and 5-8 were determined by X-ray diffraction to be composed of trinuclear metal clusters linked by stilbene dicarboxylate linkers in a paddlewheel geometry, extending to form a 2-D layered structure. In the mixed-metal cases, the larger metal ion was found to occupy the octahedral site in the cluster while the smaller ion occupies the tetrahedral positions, suggesting a selective, ligand-directed assembly process for the mixed-metal species. Variable temperature magnetic measurements for paramagnetic MOFs 3 and 6-8 were consistent with the site occupancies determined crystallographically, and indicated weak intra-cluster antiferromagnetic coupling for 3 and 8. Comparison between the crystal structures of 2, 3 and 5-8 and those reported for 1 and 4 in the literature reveal close resemblances between linker environments, with important intermolecular stilbene-stilbene geometries that are comparable in all cases. Interestingly, pale-colored 1-3 and 5-7 display very similar emission profiles upon excitation at λ(ex) = 350 nm, whereas dark-colored 4 and 8 do not exhibit detectable emission spectra. The bright, well-resolved luminescence of 1, 2 and 5 is ascribed to rigidification of the linker upon coordination to the d(10) metal ions, whereas the weaker emission observed for 3, 6 and 7 is presumably a result of quenching due to close proximity of the linker to one or more paramagnetic ions. Time-resolved measurements for 1, 2, 5 and 6 reveal biexponential emission decays, where the lifetime of the longer-lived state corresponds to observed variations in the nearest-neighbor cofacial stilbene-stilbene distances in their crystal structures. For 3, a monoexponential decay with shorter lifetime was determined, indicating significant paramagnetic quenching of its emissive state.

Local transformation of ZIF-8 powders and coatings into ZnO nanorods for photocatalytic application

Silver nitrate induces spontaneous room temperature transformation of ZIF-8 into a composite of ZnO nanorods embedded in a ZIF-8 matrix. This first example of reverse transformation of ZIF-8 into ZnO is a convenient method for generating fixed ZnO nanoparticles in powders as well as films and coatings. The fabricated ZnO nanorod@ZIF-8 is photocatalytically active.

Metal–organic frameworks as heterogeneous photocatalysts: advantages and challenges

The use of metal organic frameworks as photocatalysts is critically reviewed and their main advantages and challenges are evaluated.

A dye-sensitized Pt@UiO-66(Zr) metal–organic framework for visible-light photocatalytic hydrogen production

The rhodamine B dye was used to sensitize a Pt@UiO-66(Zr) metal–organic framework for visible-light driven photocatalytic hydrogen production from a triethanolamine aqueous solution.

Self-assembly and secondary nucleation in ZnO nanostructures derived from a lipophilic precursor

Crystal growth and self-assembly in iono-covalent nanomaterials can be directed via lipophilic interactions between a variety of metal soaps and amphiphilic lipids.

Syntheses, structures and properties of three new two-dimensional Cu(I)-Ln(III) heterometallic coordination polymers based on 2,2'-dipyridyl-5,5'-dicarboxylate ligands

Three new two-dimensional Cu(I)-Ln(III) heterometallic coordination polymers [Ln(III)Cu2(I)(Hbpdc)4] · Cl · xH2O [Ln(III) = La(III), x = 8 (1); Ln(III) = Pr(III), x=9 (2); Ln(III) = Eu(III), x = 8 (3)] (H2bpdc = 2,2'-bipyridyl-5,5'-dicarboxylic acid) have been prepared under hydrothermal conditions and structurally characterized by elemental analyses, inductively coupled plasma atomic emission spectrometry (ICP-AES) analyses, IR spectra, X-ray photoelectron spectroscopy (XPS) and single-crystal X-ray diffraction. X-ray diffraction indicates that the isomorphic 1-3 display the two-dimensional sheet structure constructed from [Cu(I)(Hbpdc)2](-) fragments through Ln(3+) connectors. Moreover, the solid-state photoluminescence measurements of 3 indicate that the Eu(III) ions, Hbpdc(-) ligands and Cu(I) cations make contributions to its luminescent properties simultaneously.

Engineering metal-based luminescence in coordination polymers and metal-organic frameworks

The rapidly growing number of luminescent coordination polymers (CPs) and metal-organic frameworks (MOFs) illustrates high interest as well as accessibility of such materials. The interplay between inorganic and organic components in these materials offers a large number of viable luminescence processes. The enormous variability of available ligand-metal combinations opens the possibility of creating luminescence "by design", based on a proper understanding of the processes involved. Together with the multifunctionality of MOFs (e.g. porosity) further options like sensing become accessible. This tutorial review covers the main luminescence processes and focuses especially on metal-based luminescence of coordination polymers and MOFs.

Studies on photocatalytic CO(2) reduction over NH2 -Uio-66(Zr) and its derivatives: towards a better understanding of photocatalysis on metal-organic frameworks

Metal-organic framework (MOF) NH2 -Uio-66(Zr) exhibits photocatalytic activity for CO2 reduction in the presence of triethanolamine as sacrificial agent under visible-light irradiation. Photoinduced electron transfer from the excited 2-aminoterephthalate (ATA) to Zr oxo clusters in NH2 -Uio-66(Zr) was for the first time revealed by photoluminescence studies. Generation of Zr(III) and its involvement in photocatalytic CO2 reduction was confirmed by ESR analysis. Moreover, NH2 -Uio-66(Zr) with mixed ATA and 2,5-diaminoterephthalate (DTA) ligands was prepared and shown to exhibit higher performance for photocatalytic CO2 reduction due to its enhanced light adsorption and increased adsorption of CO2 . This study provides a better understanding of photocatalytic CO2 reduction over MOF-based photocatalysts and also demonstrates the great potential of using MOFs as highly stable, molecularly tunable, and recyclable photocatalysts in CO2 reduction.

A luminescent metal-organic framework for sensing methanol in ethanol solution

A new luminescent Zn-MOF has been synthesized under hydrothermal condition using a semi-rigid ligand H3pcoip (4-(2-carboxyphenoxy)isophthalic acid) is reported. The luminescence properties of 1 in methanol, ethanol, and water have been investigated. Interestingly, compound 1 has a unique response to methanol compared to ethanol and water. Moreover, 1 displays a turn-on switching property triggered by methanol solvent molecules and a high sensitivity towards methanol concentration as low as 2 × 10(-7) (V(MeOH)/V(total)) in ethanol solution. The results indicate that the Zn-MOF has potential application as a sensor for detecting methanol in ethanol solution with excellent selectivity and high sensitivity.

Photocatalysis by MOFs

In this chapter it has been exemplified that MOFs can exhibit photoactivity derived from the interaction of the organic linker with the inorganic clusters. Comparison with analogous systems in solution shows that the crystal lattice of MOF, in which the linker and the organic cluster have intimate contact, originates a photochemical response that is not found for the homogeneous solution. Furthermore, MOFs can act as passive matrices to accommodate, in the internal voids, photoactive chromophores that can exhibit a distinctive response due to the properties of the surrounding media. All the discussed examples illustrate the possibility of obtaining interesting photoresponses that can open up applications beyond absorption and catalysis. Among these applications we have commented the use of MOFs as photocatalysts for environmental remediation and for the generation of solid fuels, particularly hydrogen, as well as applications in optoelectronics including solar cells and phosphors. All these applications derive from the consideration of MOFs as semiconductors.

Characterization of MOFs. 2. Long and Local Range Order Structural Determination of MOFs by Combining EXAFS and Diffraction Techniques

This chapter provides an elementary introduction to X‐ray and neutron scattering theory, written with a didactic perspective. At the beginning, the scattering process is introduced in a general way and then a differentiation between crystalline samples and amorphous samples is made, leading to the Bragg equation or to the Debye equation and to the Pair Distribution Function (PDF) approach, respectively. Advantages and disadvantages of the use of X‐rays or neutrons for scattering experiments are underlined. The basics of Extended X‐ray Absorption Fine Structure (EXAFS) spectroscopy are also reported. Starting from these basics, five examples have been selected from the recent literature where the concepts described in the first didactic part have been applied to the understanding of the structure of different MOFs materials.

Preparation of highly moisture-resistant black-colored metal organic frameworks

A straightforward method for significantly improving the moisture resistance of MOFs is described. In the proposed method, MOFs are subjected to thermal treatment, thus inducing the formation of an amorphous carbon coating on the MOF surfaces that prevents hydrolysis. This approach should open up new practical applications for MOFs in areas hitherto unexplored due to concerns regarding moisture sensitivity.

Patterning techniques for metal organic frameworks

The tuneable pore size and architecture, chemical properties and functionalization make metal organic frameworks (MOFs) attractive versatile stimuli-responsive materials. In this context, MOFs hold promise for industrial applications and a fervent research field is currently investigating MOF properties for device fabrication. Although the material properties have a crucial role, the ability to precisely locate the functional material is fundamental for device fabrication. In this progress report, advancements in the control of MOF positioning and precise localization of functional materials within MOF crystals are presented. Advantages and limitations of each reviewed technique are critically investigated, and several important gaps in the technological development for device fabrication are highlighted. Finally, promising patterning techniques are presented which are inspired by previous studies in organic and inorganic crystal patterning for the future of MOF lithography.