Enhancing the Gas Separation Selectivity of Mixed-Matrix Membranes Using a Dual-Interfacial Engineering Approach.

We report a dual-interfacial engineering approach that uses a sub-20 nm polycrystalline MOF-74 shell as a transition phase to engineer the MOF-polymer interface. The application of a shell MOF layer divides the original single interface problem into two interfaces: MOF-MOF and MOF-polymer, which can be individually addressed. The greater external surface area created by the uneven MOF-74 shell containing high-density open metal sites allows the MOF to interact with 300% polymer at the interface compared to traditional MOF, thereby ensuring good interfacial compatibility. When applied on UiO-66-NH2, its respective mixed-matrix membranes exhibit a simultaneous increase of CO2/CH4 separation selectivity and CO2 permeability with increasing MOF loading, implying a defect-free interface. When applied on MOF-801, the mixed-matrix membranes exhibit an ethylene/ethane separation selectivity up to 5.91, a drastic 76% increase compared to that of the neat polymer owing to a "gas focusing" mechanism promoted by the preferred pore orientation in the MOF-74 layer. This represents one of the most selective ethylene/ethane separation membranes reported to date.

General Way To Construct Micro- and Mesoporous Metal-Organic Framework-Based Porous Liquids.

We report a new class of porous liquids (PLs) using internally functionalized metal-organic framework (MOF) particles as pore carriers and poly(dimethylsiloxane) as bulky solvents. Using a generalizable noncovalent surface-initiated controlled radical polymerization technique, a series of isoreticular UiO-66 particles were dispersed in a liquid PDMS matrix with excellent homogeneity and colloidal stability. Benefiting from the inherent properties of PDMS, the PLs exhibit low vapor pressure, high thermal stability, and fluidity down to -35 °C. Attributed to the bulkiness of PDMS and its inherent high permeability, the sorption properties of the MOF fillers can be largely retained in their respective PLs as confirmed by low-pressure CO2, N2, Xe, and H2O sorption isotherms. The permanent porosity of the PLs can also be largely preserved even after 15 months of storage. Finally, we demonstrate that by tuning the molecular weight and polymer chain architecture of PDMS, it is possible to preserve the permanent porosity of a mesoporous MOF, MIL-101(Cr), within a PL.

Interfacial Engineering in Metal-Organic Framework-Based Mixed Matrix Membranes Using Covalently Grafted Polyimide Brushes.

Good interfacial compatibility is the key to realize the full potential of metal-organic framework-based mix matrix membranes for gas separation. Here we report a new approach that uses polyimide brushes covalently grafted on the MOF surface to engineer the MOF-polymer interface. Benefiting from the strong brush-brush interaction, polyimide grafted MOF particles can form a stand-alone membrane at 88 wt % MOF loading without the addition of polymeric matrix. Compared to traditional mixed-matrix membranes, the modified membranes exhibit improved ductility up to 472%, reduced interfacial tearing phenomenon under shear force, decreased matrix chain mobility, and improved plasticization resistance against CO2. Most importantly, with increasing MOF loading, only the modified membranes exhibit simultaneous increase of selectivity and permeability for CO2/N2 and CO2/CH4 separation, following the trend predicted by the modified Maxwell model.

Tracking and Visualization of Functional Domains in Stratified Metal-Organic Frameworks Using Gold Nanoparticles.

We report here a new technique for the identification and visualization of functional domains in stratified metal-organic frameworks (MOFs). The technique, namely, gold diffusion enabled domain identification, utilizes the diffusion of Au nanoparticles within MOF cavities to track and selectively stain the more Au-philic domain in an MOF particle thereby allowing direct observation of domains, determination of domain sequences, and, in certain cases, domain boundaries under transmission electron microscopy. This method is an excellent tool for studying MOF materials with complex domain hierarchy.

A generalizable method for the construction of MOF@polymer functional composites through surface-initiated atom transfer radical polymerization.

We report a generalizable approach to construct MOF@polymer functional composites through surface-initiated atom transfer radical polymerization (SI-ATRP). Unlike conventional SI-ATRP that requires covalent pre-anchoring of the initiating group on substrate surfaces, in our approach, a rationally designed random copolymer (RCP) macroinitiator first self-assembles on MOF surfaces through inter-chain hydrogen bond crosslinking. Subsequent polymerization in the presence of a crosslinking monomer covalently threads these polymer chains into a robust network, physically confining the MOF particle inside the polymer shell. We demonstrated the universality of this approach by growing various polymers on five MOFs of different metals (Zr, Zn, Co, Al, and Cr) with complete control over shell thickness, functionality and layer sequence while still retaining the inherent porosity of the MOFs. Moreover, the wettability of UiO-66 can be continuously tuned from superhydrophilic to superhydrophobic simply through judicious monomer(s) selection. We also demonstrated that a 7 nm polystyrene shell can effectively shield UiO-66 particles against 1 M H2SO4 and 1 M NaOH at elevated temperature, enabling their potential application in demanding chemical environments.

Uncovering two kinetic factors in the controlled growth of topologically distinct core-shell metal-organic frameworks.

Constructing core-shell metal-organic frameworks (MOFs) based on two topologically distinct MOFs is a great way to increase MOF material complexity and explore their new functions. However, such a nucleation process is energetically less favored compared to epitaxial growth due to mismatched unit cell parameters. Here, two kinetic factors, nucleation kinetics and dissolution kinetics, are revealed to be two key factors in overcoming this challenge. Through kinetic control, we demonstrate the growth of 4 types of Zr/Hf-MOF shells uniformly and contiguously on 7 different core MOFs including ZIF-8, an acid labile core. Taking advantage of the modular synthesis of Zr-MOFs, we demonstrate that post-synthetic covalent surface modification on a non-functionalizable MOF surface can be made possible through core-shell construction. We also demonstrated that the size selective catalytic behavior can be systematically tuned through changing either the ligand length or ligand functionality.

Adhesive bacterial amyloid nanofiber-mediated growth of metal-organic frameworks on diverse polymeric substrates.

The development of a simple, robust, and generalizable approach for spatially controlled growth of metal-organic frameworks (MOFs) on diverse polymeric substrates is of profound technological significance but remains a major challenge. Here, we reported the use of adhesive bacterial amyloid nanofibers, also known as curli nanofibers (CNFs), major protein components of bacterial biofilms, as universal and chemically/mechanically robust coatings on various polymeric substrates to achieve controlled MOF growth with improved surface coverage up to 100-fold. Notably, owing to the intrinsic adhesive attributes of CNFs, our approach is applicable for MOF growth on both 2D surfaces and 3D objects regardless of their geometric complexity. Applying this technique to membrane fabrication afforded a thin-film composite membrane comprising a 760 ± 80 nm ZIF-8 selective layer grown on a microporous polyvinylidene fluoride (PVDF) support which exhibited a C3H6/C3H8 mixed-gas separation factor up to 10, C3H6 permeance up to 1110 GPU and operational stability up to 7 days. Our simple yet robust approach therefore provides new insights into designing new interfaces for mediating MOF growth and opens new opportunities for constructing new MOF-based membranes and devices.

In Situ Real-Time Study of the Dynamic Formation and Conversion Processes of Metal Halide Perovskite Films.

Metal halide perovskite solar cells (PSCs) have advanced to the forefront of solution-processed photovoltaic techniques and made stunning progress in power conversion efficiency (PCE). Further improvements in device performances rely on perfecting the structure and morphology of perovskite films. However, undesirable defects such as pinholes and grain boundaries are often created in film preparations due to lack of knowledge of the precise reaction mechanism. Here, in situ grazing-incidence X-ray diffraction (GI-XRD) investigations are performed, facilitated by other techniques, on the formation of the widely adopted MAPbI3 (MA = methylammonium) perovskite films from their intermediate adduct (IA) phases. The influences of solvent vapor atmospheres on MAPbI3 films are also systematically investigated, where the dynamic conversion processes between different phases are visualized in real time. Further in situ GI-XRD and infrared spectroscopy measurements reveal that the IA phases contain both N,N-dimethylformamide and dimethyl sulfoxide (DMSO) as coordinating molecules. By tuning the DMSO concentration in perovskite precursors, the ideal perovskite film is formed and the best PCE is achieved for the planar MAPbI3 -based PSCs. These findings highlight the role of IA phases and the effect of solvent atmospheres on the quality of perovskite films, providing direct insights into their growth mechanism.