The high porosity of metal-organic frameworks (MOFs) has been used to achieve exceptional gas adsorptive properties but as yet remains largely unexplored for electrochemical energy storage devices. This study shows that MOFs made as nanocrystals (nMOFs) can be doped with graphene and successfully incorporated into devices to function as supercapacitors. A series of 23 different nMOFs with multiple organic functionalities and metal ions, differing pore sizes and shapes, discrete and infinite metal oxide backbones, large and small nanocrystals, and a variety of structure types have been prepared and examined. Several members of this series give high capacitance; in particular, a zirconium MOF exhibits exceptionally high capacitance. It has the stack and areal capacitance of 0.64 and 5.09 mF cm(-2), about 6 times that of the supercapacitors made from the benchmark commercial activated carbon materials and a performance that is preserved over at least 10000 charge/discharge cycles.

Materials development for artificial photosynthesis, in particular, CO reduction, has been under extensive efforts, ranging from inorganic semiconductors to molecular complexes. In this report, we demonstrate a metal-organic framework (MOF)-coated nanoparticle photocatalyst with enhanced CO reduction activity and stability, which stems from having two different functional units for activity enhancement and catalytic stability combined together as a single construct. Covalently attaching a CO-to-CO conversion photocatalyst Re(CO)(BPYDC)Cl, BPYDC = 2,2'-bipyridine-5,5'-dicarboxylate, to a zirconium MOF, UiO-67 (Re-MOF), prevents dimerization leading to deactivation. By systematically controlling its density in the framework (n = 0, 1, 2, 3, 5, 11, 16, and 24 complexes per unit cell), the highest photocatalytic activity was found for Re-MOF. Structural analysis of Re-MOFs suggests that a fine balance of proximity between photoactive centers is needed for cooperatively enhanced photocatalytic activity, where an optimum number of Re complexes per unit cell should reach the highest activity. Based on the structure-activity correlation of Re-MOFs, Re-MOF was coated onto Ag nanocubes (Ag⊂Re-MOF), which spatially confined photoactive Re centers to the intensified near-surface electric fields at the surface of Ag nanocubes, resulting in a 7-fold enhancement of CO-to-CO conversion under visible light with long-term stability maintained up to 48 h.

We show that the activity and selectivity of Cu catalyst can be promoted by a Zr-based metal-organic framework (MOF), ZrO(OH)(BDC) (BDC = 1,4-benzenedicarboxylate), UiO-66, to have a strong interaction with Zr oxide [ZrO(OH)(-CO)] secondary building units (SBUs) of the MOF for CO hydrogenation to methanol. These interesting features are achieved by a catalyst composed of 18 nm single Cu nanocrystal (NC) encapsulated within single crystal UiO-66 (Cu⊂UiO-66). The performance of this catalyst construct exceeds the benchmark Cu/ZnO/AlO catalyst and gives a steady 8-fold enhanced yield and 100% selectivity for methanol. The X-ray photoelectron spectroscopy data obtained on the surface of the catalyst show that Zr 3d binding energy is shifted toward lower oxidation state in the presence of Cu NC, suggesting that there is a strong interaction between Cu NC and Zr oxide SBUs of the MOF to make a highly active Cu catalyst.

Chemical environment control of the metal nanoparticles (NPs) embedded in nanocrystalline metal-organic frameworks (nMOFs) is useful in controlling the activity and selectivity of catalytic reactions. In this report, organic linkers with two functional groups, sulfonic acid (-SO3H, S) and ammonium (-NH3(+), N), are chosen as strong and weak acidic functionalities, respectively, and then incorporated into a MOF [Zr6O4(OH)4(BDC)6 (BDC = 1,4-benzenedicarboxylate), termed UiO-66] separately or together in the presence of 2.5 nm Pt NPs to build a series of Pt NPs-embedded in UiO-66 (Pt⊂nUiO-66). We find that these chemical functionalities play a critical role in product selectivity and activity in the gas-phase conversion of methylcyclopentane (MCP) to acyclic isomer, olefins, cyclohexane, and benzene. Pt⊂nUiO-66-S gives the highest selectivity to C6-cyclic products (62.4% and 28.6% for cyclohexane and benzene, respectively) without acyclic isomers products. Moreover, its catalytic activity was doubled relative to the nonfunctionalized Pt⊂nUiO-66. In contrast, Pt⊂nUiO-66-N decreases selectivity for C6-cyclic products to <50% while increases the acyclic isomer selectivity to 38.6%. Interestingly, the Pt⊂nUiO-66-SN containing both functional groups gave different product selectivity than their constituents; no cyclohexane was produced, while benzene was the dominant product with olefins and acyclic isomers as minor products. All Pt⊂nUiO-66 catalysts with different functionalities remain intact and maintain their crystal structure, morphology, and chemical functionalities without catalytic deactivation after reactions over 8 h.

Generally, crystals of synthetic porous materials such as metal-organic frameworks (MOFs) are commonly made up from one kind of repeating pore structure which predominates the whole material. Surprisingly, little is known about how to introduce heterogeneously arranged pores within a crystal of homogeneous pores without losing the crystalline nature of the material. Here, we outline a strategy for producing crystals of MOF-5 in which a system of meso- and macropores either permeates the whole crystal to make sponge-like crystals or is entirely enclosed by a thick crystalline microporous MOF-5 sheath to make pomegranate-like crystals. These new forms of crystals represent a new class of materials in which micro-, meso-, and macroporosity are juxtaposed and are directly linked unique arrangements known to be useful in natural systems but heretofore unknown in synthetic crystals.

The growth of nanocrystalline metal-organic frameworks (nMOFs) around metal nanocrystals (NCs) is useful in controlling the chemistry and metric of metal NCs. In this Letter, we show rare examples of nMOFs grown in monocrystalline form around metal NCs. Specifically, Pt NCs were subjected to reactions yielding Zr(IV) nMOFs [Zr6O4(OH)4(fumarate)6, MOF-801; Zr6O4(OH)4(BDC)6 (BDC = 1,4-benzenedicarboxylate), UiO-66; Zr6O4(OH)4(BPDC)6 (BPDC = 4,4'-biphenyldicarboxylate), UiO-67] as a single crystal within which the Pt NCs are embedded. These constructs (Pt⊂nMOF)nanocrystal are found to be active in gas-phase hydrogenative conversion of methylcyclopentane (MCP) and give unusual product selectivity. The Pt⊂nUiO-66 shows selectivity to C6-cyclic hydrocarbons such as cyclohexane and benzene that takes place with 100 °C lower temperature than the standard reaction (Pt-on-SiO2). We observe a pore size effect in the nMOF series where the small pore of Pt⊂nMOF-801 does not produce the same products, while the larger pore Pt⊂nUiO-67 catalyst provides the same products but with different selectivity. The (Pt⊂nMOF)nanocrystal spent catalyst is found to maintain the original crystallinity, and be recyclable without any byproduct residues.

Graphical abstract

Flexible perovskite solar cells (PSCs) have attracted considerable attention due to their excellent performance, low-cost, and great potential as an energy supplier for soft electronic devices. In particular, the design of charge transporting layers (CTLs) is crucial to the development of highly efficient and flexible PSCs. Herein, nanocrystalline Ti-based metal-organic framework (nTi-MOF) particles are synthesized to have ca. 6 nm in diameter. These are then well-dispersed in alcohol solvents in order to generate electron transporting layers (ETLs) in PSCs under ambient temperatures using a spin-coating process. The electronic structure of nTi-MOF ETL is found to be suitable for charge injection and transfer from the perovskite to the electrodes. The combination of a [6,6]-phenyl-C-butyric acid (PCBM) into the nTi-MOF ETL provides for efficient electron transfer and also suppresses direct contact between the perovskite and the electrode. This results in impressive power conversion efficiencies (PCEs) of 18.94% and 17.43% for rigid and flexible devices, respectively. Moreover, outstanding mechanical stability is retained after 700 bending cycles at a bending radius ( r) of 10 mm.