ZIF-8 Films Research Articles

Porosimetry for Thin Films of Metal-Organic Frameworks: A Comparison of Positron Annihilation Lifetime Spectroscopy and Adsorption-Based Methods.

Thin films of crystalline and porous metal-organic frameworks (MOFs) have great potential in membranes, sensors, and microelectronic chips. While the morphology and crystallinity of MOF films can be evaluated using widely available techniques, characterizing their pore size, pore volume, and specific surface area is challenging due to the low amount of material and substrate effects. Positron annihilation lifetime spectroscopy (PALS) is introduced as a powerful method to obtain pore size information and depth profiling in MOF films. The complementarity of this approach to established physisorption-based methods such as quartz crystal microbalance (QCM) gravimetry, ellipsometric porosimetry (EP), and Kr physisorption (KrP) is illustrated. This comprehensive discussion on MOF thin film porosimetry is supported by experimental data for thin films of ZIF-8.

Facile in situ growth of ZIF-8 films onto aluminum for applications requiring fast thermal response

In this work, we report a simple approach for the synthesis of highly crystalline films of ZIF-8 directly grown onto non-functionalized common aluminum substrates through an easy, short in situ solvothermal methodology. Through several techniques such as XRD, SEM, EDS, AFM and XPS, it is shown that a simple, short pretreatment of the substrate with a dilute hydrochloric acid solution promotes the subsequent solvothermal nucleation and growth of continuous, homogeneous micrometer thickness and very adherent ZIF-8 films. The robustness and usefulness of ZIF-8/aluminum are shown by testing it in the highly exothermic catalytic reaction of carbon monoxide oxidation, in which it presented a high performance and durability, preserving the metal–organic framework integrity during reaction time. This behavior demonstrates that the novel ZIF-8/aluminum systems have a high potential for use in applications at moderate temperatures demanding a fast heat exchange rate between the metal–organic framework (MOF) film and the substrate.

ZIF-8 engineered bismuth nanosheet arrays for boosted electrochemical reduction of nitrate.

Removal of nitrate in wastewater is of great importance to environmental protection and humanity. However, the competitive reaction of hydrogen evolution (HER), which could occupy most active sites of the electrocatalyst, is one of the big challenges for nitrate removal. In this study, a novel zeolitic imidazolate framework-8 film engineered bismuth nanosheet electrocatalyst (ZIF-8/Bi-CC) was designed and synthesized for the electrochemical reduction of nitrate. The water contact angle and electrochemical tests demonstrated that the construction of the hydrophobic ZIF-8 film effectively weakened the competition of HER. And the nitrate removal efficiency and ammonium selectivity increased by 25.9% and 34.2% respectively after bismuth nanosheets were embedded into the ZIF-8 film. Besides, the bismuth concentration detection results indicated that the ZIF-8 film as the protective shell could effectively prevent the leaching of bismuth into the solution. More importantly, the final nitrate removal rate of ZIF-8/Bi-CC was close to 90% after 5 h when treating actual garbage fly ash wastewater, the NITRR efficiency stability and the obtained product were confirmed by five electrochemical cycles. The metal-organic framework film engineered electrocatalyst is a promising strategy for designing a new catalyst for the removal of nitrate in industrial wastewater.

An enhanced fluorescent ZIF-8 film by capturing guest molecules for light-emitting applications

ZIF-8 film with enhanced fluorescence is successfully synthesized via capturing CH3COOH as guest molecules, and the PLQY of the ZIF-8 composite film reaches 54.42%.

Effect of different oxide and hybrid precursors on MOF-CVD of ZIF-8 films.

Chemical vapor deposition of metal-organic frameworks (MOF-CVD) will facilitate the integration of porous and crystalline coatings in electronic devices. In the two-step MOF-CVD process, a precursor layer is first deposited and subsequently converted to a MOF through exposure to linker vapor. We herein report the impact of different metal oxide and metalcone layers as precursors for zeolitic imidazolate framework ZIF-8 films.

Flexible Polypropylene-Supported ZIF-8 Membranes for Highly Efficient Propene/Propane Separation.

Metal-organic framework (MOF) membranes have enormous potential in separation applications. There are several MOF membranes grown on polymer substrates aimed for scale-up, but their brittleness hampers any industrial application. Herein, intergrown continuous polypropylene (PP)-supported ZIF-8 membranes have been successfully synthesized via fast current-driven synthesis (FCDS) within 1 h. The PP-supported ZIF-8 membranes exhibit a promising separation factor of 122 ± 13 for binary C3H6-C3H8 mixtures combined with excellent flexibility behavior. The C3H6/C3H8 separation performance of the PP-supported ZIF-8 membrane was found to be constant after bending the supported ZIF-8 film with a curvature of 92 m-1. This outstanding mechanical property is crucial for practical applications. Moreover, we further synthesized ZIF-8 membranes on various polymer substrates and even polymer hollow fibers to demonstrate the production scalability.

One-pot synthesis of core–shell ZIF-8@ZnO porous nanospheres with improved ethanol gas sensing

Rational design and synthesis of hollow core–shell hetero-structures with high complexity still remains challenging for high gas-sensing performances. Here, ZIF-8@ZnO (zeolite imidazolate framework-8) core–shell material was synthesized using an easy-to-use hydrothermal reaction. ZnO is a kind of homologous metal oxide of ZIF-8 material. Therefore, without adding other zinc salts, the formation of ZIF-8@ZnO was completed in a one-pot reaction. Then a series of morphology and structure analysis including XRD, BET, SEM, EDX, etc. were carried out to study the positive effect of ZIF-8 film on the sensing performance of ZnO nanospheres. The results show that the ZIF-8 film uniformly covers the surface of porous ZnO nanospheres with a diameter of about 400 nm, forming a core–shell structure. Compared to the other volatile organic compounds (VOCs), ZIF-8@ZnO shows a higher sensitivity and selectivity to 50 ppm ethanol gas at an operating temperature of 220 °C, which can achieve 13.8. This is almost 2.7 times that of pure ZnO nanospheres (5.1). The obtained results show that ZIF-8@ZnO core–shell material not only greatly improves the sensitivity of the sensor, but also enhance the selectivity of ethanol gas sensing.

One-step in-situ growth of zeolitic imidazole frameworks-8 on cotton fabrics for photocatalysis and antimicrobial activity

Multifunctional textiles were fabricated by one-step in-situ synthesis of zeolite imidazole skeleton-8 (ZIF-8) on polydopamine (pDA) templated cotton fabrics. A homogenous distribution of ZIF-8 nanoparticles on cotton fiber surface was achieved as per surface morphology study by SEM and TEM, and the heterogeneous nucleation mechanism of depositing the dense ZIF-8 film on pDA-templated cotton fabrics was proposed. FTIR spectra, XPS, and XRD have indicated the presence and confirmed the chemical composition of ZIF-8 on coated cotton fabrics. The fabricated ZIF-8/cotton fabrics showed excellent photocatalysis for Methylene blue decomposition under ultraviolet radiation, and the photocatalysis mechanism was discussed. In addition, the coated fabrics showed robust antimicrobial activity towards Escherichia coli. This facile and affordable fabrication method based on dopamine and ZIF-8 is promising for large-scale production of multifunctional textiles.

Micrometer scale pore-interconnectivity in nanoporous ZIF-8 films with Zn enriched surface terminations

Abstract ZIF-8 nanoporous films of variable thicknesses, which are proposed to be very low-k dielectric films for chip industries, have been deposited on Si substrate using simple solution chemistry method. Contrary to the earlier reports, ZIF-8 crystals number density on the substrate in early deposition cycles increases leading to higher coverage of the substrate. The growth of crystal size or film thickness occurs in later deposition cycles. The deposition of ZIF-8 crystals is highly random, however a preferential growth in 002 orientation is observed. Synchrotron based X-ray photoelectron spectroscopy has confirmed Zn enrichment at surface of the films compared to imidazole linkers. Depth dependent Doppler broadening spectroscopy measurements utilizing the positronium diffusion process have unequivocally confirmed that the pores in ZIF-8 films are interconnected at micrometer scale leading to their accessibility from the outer surface. This type of pore architecture along with the surface enrichment might be responsible for adsorbate selectivity and very high adsorption capacity of these films as reported in literature. On thermal annealing, the pore architecture of these films begins to collapse at a much lower temperature (~373 K) than the decomposition temperature (~700 K) of ZIF-8 powder.

Improved performance of 3D hierarchical NiAl-LDHs micro-flowers via a surface anchored ZIF-8 for rapid multiple-pollutants simultaneous removal and residues monitoring

In this paper, we report a new type of 3D ZIF-8@NiAl-LDHs micro-flowers material consisting of sandwich-like structured 2D nanopetals (highly compact ZIF-8 film anchored on both sides of petals). ZIF-8 was successfully incorporated into NiAl-LDHs (ZIF-8@NiAl-LDHs) via seeding strategy directed growth of ZIFs on the surface of LDHs nanopetals. The coating of ZIF-8 significantly increased the adsorption ability to organic pollutants and inorganic cation. 3D ZIF-8@NiAl-LDHs with excellent enrichment and filtration properties has been exploited for the application in water purification, and exhibit superior high adsorption rate and adsorption efficiency of organic (nonsteroidal anti-inflammatory drugs: ketoprofen, flurbiprofen, indometacin and ibuprofe; anionic dyes: congo red, orange g; cationic dyes: methylene blue, rhodamine b) and inorganic cation (Cu2+, Pb2+) residues due to their novel hierarchical and submicroscopic structures. Further, 3D ZIF-8@NiAl-LDHs as filter membrane to extraction four kind of trace anti-inflammatory drugs followed by direct quantification detection of targets with HPLC was demonstrated. The validated method was successfully applied for analysis of four anti-inflammatory drugs in environmental water and human urine samples. This work provided a feasible way to design and construct purification materials for wastewater treatment and contaminant detection.

Structural characterization and optical properties of zeolitic imidazolate frameworks (ZIF-8) for solid-state electronics applications

Zeolitic imidazolate frameworks (ZIF-8) thin films were fabricated via layer by a layer deposition method. The investigation of X-ray diffraction (XRD) of the ZIF-8 powder form was seen to be polycrystalline with a single cubic-phase structure. The lattice parameters and Miller index were calculated using Rietveld refinement by Fullprof software. XRD of ZIF-8 film demonstrates an amorphous structure. The morphology of the ZIF-8 films has therefore been studied with the scanning laser microscope method and found that the homogeneity and surface enhance with increasing the film thickness. On the other hand, it is found that ZIF-8 films have a direct electronic transition with two energy bandgaps (3.62 and 4.58 eV) and their values independent of the thickness of the films. The optical constants were calculated, and the refractive index esteems increment with expanding of the film thickness. This outcome might be credited to the interlude of the film in the preliminary phase of the deposition. The nonlinear optical constants for ZIF-8 film were calculated, and the third-order optical nonlinearity was expanded with expanding the film thickness.

Post-Synthetic Modification of ZIF-8 Crystals and Films through UV Light Photoirradiation: Impact on the Physicochemical Behavior of the MOF.

The physicochemical modification of Metal-Organic Frameworks (MOFs) is a current challenge in the search to improve their performance in different technological applications. In this work we analyze the post-synthetic modification of ZIF-8 crystals and films through a simple and clean treatment that involves the exposure to a UV lamp under environmental conditions. It is demonstrated that a short treatment alters the MOF structure and chemistry, providing a modified ZIF-8 due to partial disconnections of its structure which increase the amount of terminal surface species such as Zn-OH and -C=N-H, but without compromising the overall MOF structure, specific surface area or thermal stability. Additionally, it leads to changes in several properties of the ZIF-8, such as its capacity to accumulate charge through pseudocapacitive processes, its interaction with nitric oxide and its light absorption behavior. This strategy of modifying ZIF-8 without the use of chemicals through a gentle disconnection of its own structure could open new perspectives of post-functionalization of crystals and films of ZIF-8 to be used in a wide range of applications.

Vapor-deposited zeolitic imidazolate frameworks as gap-filling ultra-low-k dielectrics

The performance of modern chips is strongly related to the multi-layer interconnect structure that interfaces the semiconductor layer with the outside world. The resulting demand to continuously reduce the k-value of the dielectric in these interconnects creates multiple integration challenges and encourages the search for novel materials. Here we report a strategy for the integration of metal-organic frameworks (MOFs) as gap-filling low-k dielectrics in advanced on-chip interconnects. The method relies on the selective conversion of purpose-grown or native metal-oxide films on the metal interconnect lines into MOFs by exposure to organic linker vapor. The proposed strategy is validated for thin films of the zeolitic imidazolate frameworks ZIF-8 and ZIF-67, formed in 2-methylimidazole vapor from ALD ZnO and native CoOx, respectively. Both materials show a Young’s modulus and dielectric constant comparable to state-of-the-art porous organosilica dielectrics. Moreover, the fast nucleation and volume expansion accompanying the oxide-to-MOF conversion enable uniform growth and gap-filling of narrow trenches, as demonstrated for 45 nm half-pitch fork-fork capacitors.

High performance nanoporous carbon microsupercapacitors generated by a solvent-free MOF-CVD method

Micro power sources that support higher power capabilities, specific energies, and ultra-long cycling lifetimes are required for next generation microelectro-mechanical systems (MEMS) and miniaturized portable microdevices. We demonstrated a simple, solvent-free method to coat interdigitated gold electrodes with a conformal layer of nanoporous carbon. Initially the electrodes were coated with a film of ZnO via electrodeposition to serve as the zinc source. These films were exposed to 2-methylimidazole vapor to transform the ZnO into a metal organic framework (MOF) called ZIF-8. The thickness of the ZnO film and the duration of the MOF conversion process created ZIF-8 films with different morphologies, cross-sections and elemental compositions. The ZIF-8 films were then carbonized at 800 °C under a flowing environment of N2 gas to generate nanoporous carbon film electrodes for supercapacitors. The devices have superior capacitance retention, in addition to high power and energy densities. Our 16-electrode microsupercapacitor had a stack power of 232.8 W⋅cm−3, which could be attained even at a scan rate of 1000 V ⋅s−1, without rapid stack energy attenuation. The process of combining an inexpensive ZnO electrodeposition method with solvent-free MOF deposition and subsequent carbonization generates high surface area carbon electrodes with good electrochemical performance.

High electrocatalytic reduction using ZnS micropolyhedron: Direct sulfuration of ZIF-8 film on conducting glass by chemical vapour deposition

The micropolyhedral ZnS film on fluorine-doped tin oxide (FTO) glass was prepared as following: ZIF-8 film on FTO by dipping method and then directly sulfuretted by chemical vapour deposition. Compared with ZnS particles, ZnS micropolyhedrons demonstrated excellent electrocatalytic reduction from triiodide to iodide as a counter electrode due to ZIF-8 skeleton. The ordered polyhedrons achieved lower electron transport impedance than that of the chaotic particle structure. The assumption can be confirmed by the electrocatalytic reduction from triiodide to iodide.

Self-crystallization of uniformly oriented zeolitic imidazolate framework films at air-water interfaces.

Gas-liquid interfaces with unique physicochemical properties have great potential for the self-assembly of many materials. Herein, a concept of the autonomous self-crystallization of MOF films at air-water interfaces is reported. The free-standing ZIF-8 films with a large area of about 20 cm2 can be preferentially assembled only at the water surface. Under the influence of the atomically well-defined and amphiphilic interface on anisotropically polar linkers, the thus-prepared ZIF-8 films exhibit highly out-of-plane orientation and smooth-rough Janus crystalline facets.

Zeolitic imidazolate framework ZIF-8 films by ZnO to ZIF-8 conversion and their usage as seed layers for propylene-selective ZIF-8 membranes

Supported ZIF-8 films with varying microstructures are prepared by ZnO to ZIF-8 conversion reaction in a 2-methylimidazole ligand solution. The grain size and continuity of the films are controlled by manipulating two critical conversion parameters such as solvents and their blends, and temperature. The ZIF-8 films with varying microstructures are used as seed layers to synthesize propylene-selective ZIF-8 membranes. The propylene/propane separation performances are greatly affected by the seed layer microstructures. Non-continuous seed layers with compactly packed ZIF-8 nanocrystals tend to yield more propylene-selective membranes than those grown from continuous seed layers with larger grain size likely due to improved grain boundary structure. Furthermore, the membranes are mechanically stronger than those grown from conventional seed layers, which are obtained by dip-coating ZIF-8 nanocrystals on substrates, possibly owing to the presence of mechanical interlocks between the membranes and porous substrate network.

A differential photoelectrochemical hydrogen peroxide sensor based on catalytic activity difference between two zeolitic imidazolate framework surface coatings

Improvement of the sensitivity and selectivity is important in the development of an analytical method. Herein, we propose a differential photoelectrochemical (PEC) sensor to hydrogen peroxide based on the catalytic activity difference between two zeolitic imidazolate framework (ZIF) surface coatings. In the sensing and reference photoelectrodes, ZIF-67 and ZIF-8 films were grown on the surface of carboxylated carbon nanotubes-graphite-like carbon nitride/TiO2 nanotube array, respectively. The molecular sieve effect of ZIF-8 and ZIF-67 coatings can prevent the diffusion of molecules with size larger than the pores in coating layers (e.g. ascorbic acid), improving the selectivity of the PEC sensor to H2O2. The sensing photoelectrode offers higher sensitivity to H2O2 than the reference one due to the excellent catalytic activity of ZIF-67. The interference from the surfaces of the sensing and reference photoelectrodes can be eliminated effectively by the differential strategy, enhancing further the selectivity, reliability and stability of the sensor. Under optimized conditions, the as-prepared differential PEC sensor was applied to the detection of H2O2 with limit of detection of 1.5 nM, exhibiting high sensitivity and anti-interference ability.

Heterogeneous growth of continuous ZIF-8 films on low-temperature amorphous silicon

Thin amorphous silicon films, deposited at low temperature by Inductively Coupled Plasma Chemical Vapor Deposition, have, for the first time, been employed as substrate for ZIF-8 growth. In order to investigate the role of the surface chemistry on the nucleation process, films have also been grown on other silicon-based substrates such as H-terminated Si(1 0 0), SiO2 and quartz. Film preparation was carried out at room temperature using a mixed Zn nitrate and imidazole solution in methanol or ethanol. Using methanol, continuous ZIF-8 films were obtained on amorphous Si and H-terminated Si(1 0 0), while less homogeneous films were formed on the other surfaces. In ethanol, slower growth rates occurred and thinner films, compared to the ones in methanol, were obtained. These slower rates highlight the different effects of the four surfaces on the growth process. These differences have been related to the silanol density of the surfaces and to the Lewis basic strength which affect imidazole moiety deprotonation. H-terminated Si(1 0 0) and amorphous Si turned out to be the most reactive surfaces, whereas on quartz and, especially, on SiO2 reactivity was much lower. Experimental results have been validated by the DFT modelling of the proton exchange, which takes place between the imidazole group and the surface. Finally, the VOCs adsorption capability of ZIF-8 films grown on amorphous silicon has been evaluated through temperature desorption experiments.

Shedding Light on the Dark Corners of Metal-Organic Framework Thin Films: Growth and Structural Stability of ZIF-8 Layers Probed by Optical Waveguide Spectroscopy.

Metal-organic framework (MOF) thin films are promising materials for multiple technological applications, such as chemical sensing. However, one potential limitation for their widespread use in different settings is their stability in aqueous environments. In the case of ZIF-8 (zeolitic imidazolate framework) thin films, their stability in aqueous media is currently a matter of debate. Here, we show that optical waveguide spectroscopy (OWS), in combination with surface plasmon resonance (SPR) spectroscopy, offers a convenient way for answering intriguing questions related to the stability of MOF thin films in aqueous solutions and, eventually provide a tool for assessing changes in MOF layers under different environmental conditions. Our experiments relied on the use of ZIF-8 thin films grown on surface-modified gold substrates, as optical waveguides. We have found a linear thickness increase after each growing cycle and observed that the growing characteristics are strongly influenced by the nature of the primer layer. One of our findings is that substrate surface modification with a 3-mercapto-1-propanesulfonate (MPSA) primer layer is critical to achieve ZIF-8 layers that can effectively act as optical waveguides. We observed that ZIF-8 films are structurally stable upon exposure to pure water and 50 mM NaCl solutions but they exhibit a slight swelling and an increase in porosity probably due to the permeation of the solvent in the intergrain mesoporous cavities. However, OWS revealed that exposure of ZIF-8 thin films to phosphate-buffered saline solutions (pH 8) promotes significant film degradation. This poses an important question as to the prospective use of ZIF-8 materials in biologically relevant applications. In addition, it was demonstrated that postsynthetic polyelectrolyte modification of ZIF-8 films has no detrimental effects on the structural stability of the films.