ZIF-8 Thin Films Research Articles

Surface plasmon resonance sensor of hollow microstructure fiber based on ZIF-8 thin film

An annular hollow microstructured fiber surface plasmon resonance (SPR) sensor based on ZIF-8 film is proposed for the measurement of CO2 concentration in air. Internal detection is adopted to enhance the spectral response to CO2 gas, and nano-gold pillars are embedded on both sides of the channel. The geometric parameters of the sensor are optimized to achieve an average sensitivity of 1.01 nm/% over the range of CO2 concentration from 0% to 100%. The results indicate that the sensor is a promising SPR gas sensor with great potential in CO2 detection applications.

ZIF-8 thin films by a vapor-phase process: limits to growth.

Metal-organic frameworks are a class of porous materials that show promising properties in the field of microelectronics. To reach industrial use of these materials, gas phase techniques are often preferred and were recently introduced. However, the thicknesses achieved are not sufficient, limiting further development. In this work, an improved gas phase process allowing ZIF-8 layer formation of several hundreds of nm using cyclic ligand/water exposures is described. Then, by a combination of in-depth surface analyses and molecular dynamics simulations, the presence and role of hydroxyl defects in the ZIF-8 layer to reach this thickness are established. At the same time, this study unveils an inherent limit of the method: thickness growth is consubstantial with defect repairing upon the crystallites ripening; such defect repairing eventually leads to the decrease of the pore window below the diffusion radius of the incoming linker, thus apparently capping the maximum MOF thickness observable for this type of material topology through this growth method.

A novel approach towards controlled growth of metal-organic framework ZIF-8 thin film on steel with excellent corrosion protection

The control over the in-situ growth of metal-organic framework (MOF) thin films on a metal surface is still challenging. As an interesting subdivision of MOFs, the zeolite imidazole framework (ZIF) has been the research hotspot in various research areas, from separation to sensing in the form of nanocrystals, thin films, and membranes. Continuous thin films of ZIF crystals need binding sites for desirable heterogeneous nucleation. This study proposed a novel and facile surface treatment at room temperature based on zinc-phosphate conversion coating to provide nucleation sites for the intergrown ZIF-8 crystals on mild steel as a corrosion protective layer. The successful growth of ZIF-8 crystals on the modified carbon steel was characterized using FT-IR, Raman, GIXRD, XPS, and FE-SEM tests. This thin layer led to a super protective barrier film for up to 10 days in a harsh corrosive environment of salt spray. On the other hand, EIS reported almost 100 kΩ.cm2 total resistance after 72 h immersion in 3.5% wt. NaCl solution which is a great achievement compared to conventional primer coatings. The total resistance of an epoxy coating with the proposed primer was improved by 135% when there was a deep mechanical scratch in the coating. The ZIF-8 thin film could significantly improve the epoxy coating's anti-corrosion ability. The good news is that this concept is transferable to other ZIF-based MOFs.

Accessing accelerated molecular diffusion by nanopore alignment in a MOF thin film

By aligning the pore orientation in the ZIF-8 thin film molecular transport diffusivity could be enhanced by 2-fold as compared to a mixed orientation ZIF-8 thin film.

Large refractive index changes in ZIF-8 thin films of optical quality.

ZIF-8 (zeolitic imidazolate framework-8) is a member of the growing family of metal–organic frameworks (MOFs). Homogeneous and crack-free ZIF-8 thin films of optical quality were crystallized on silicon and glass substrates. The refractive index of such ZIF-8 thin films in the complete visible light spectrum was directly determined for the first time. By incubating the porous films in different substances, the refractive index could be modulated over a wide range, two times larger than previously reported for MOF thin films. Reversible refractive index switching in ZIF-8 thin films was performed via the liquid and the gas phase. The ability to adjust the refractive index over a broad range enables the use of ZIF-8 films for applications in optical devices such as sensors, coatings for mirrors and lenses, or as an optical medium in more complex optical devices.

A novel ratiometric fluorescent thermometer based on ZIF-8 thin films

A ratiometric fluorescent thermometer (RPE/ZIF-8) is prepared by embedding red-emitting fluorescent proteins (R-phycoerythrin, RPE) into zeolitic imidazolate frameworks (ZIF-8) through a solid-confinement process. After encapsulated into ZIF-8 crystals, RPE proteins are uniformly distributed in the composite membrane. The resulting RPE/ZIF-8 thin film has three fluorescence emission peaks at 520, 605 and 650 nm, whose fluorescent intensity will decrease regularly as environment temperature increasing. This fluorescent thermometer has wide measurement range and excellent sensitivity, which can be applied for some low temperature detections after assembled into a portable device.

Large-Area Crystalline Zeolitic Imidazolate Framework Thin Films.

We report that continuous MOF films with highly controlled thickness (from 44 to 5100 nm) can be deposited over length scales greater than 80 centimeters by a facile, fast, and cost-effective spray-coating method. Such success relies on our discovery of unprecedented perfectly dispersed colloidal solutions consisting of amorphous MOF nanoparticles, which we adopted as precursors that readily converted to the crystalline films upon low-temperature in situ heating. The colloidal solutions allow for the fabrication of compact and uniform MOF films on a great deal of substrates such as fluorine-doped tin oxide, glass, SiO2 , Al2 O3 , Si, Cu, and even flexible polycarbonate, widening their technological applications where substrates are essential. Despite the present work focuses on the fabrication of uniform cobalt-(2-methylimidazole)2 and zinc-(2-methylimidazole)2 films, our findings mark a great possibility in producing other high-quality MOF thin films on a large scale.

Plant polyphenol intermediated metal-organic framework (MOF) membranes for efficient desalination

Synthesis of a continuous and intergrown metal-organic framework (MOF) membrane remains a challenging issue due to the grain boundary defects and intercrystalline cracks in MOF thin film. In this study, a ZIF-8 membrane was fabricated by using a plant polyphenol tannic acid (TA) layer as an intermediate layer to overcome this issue. Specifically, a TA layer was constructed on a polyethersulfone substrate via a facile and environmentally friendly aqueous coating process. Thereafter, the above modified membrane was immersed into a Zn(Ac)2 and Hmim mixture solution for in-situ growing a ZIF-8 thin film on membrane surface. It was found that the abundant pyrogallol groups in TA layer could coordinate with Zn2+ ions, promote heterogeneous nucleation and contribute to a continuous and intergrown ZIF-8 thin film. The as-prepared ZIF-8 membrane showed a pure water permeance of 3.6 L m−2 h−1 bar−1 with NaCl and Na2SO4 rejections of 64.7% and 92.2% respectively, overwhelming the controlled ZIF-8 membrane without TA as intermediate layer in both permeability and selectivity. This membrane is one of few MOF membranes that have a divalent salt rejection higher than 90%, demonstrating its great potential application in efficient desalination.

Nanofiltration of dye solution using chitosan/poly(vinyl alcohol)/ZIF-8 thin film composite adsorptive membranes with PVDF membrane beneath as support

Nanobiocomposite adsorptive membranes were engineered by integrating metal-organic frameworks (ZIF-8), into the chitosan/poly(vinyl alcohol) blend and coating the blend on PVDF membrane support for an enhanced separation performance. The membranes were characterized by SEM, AFM, BET, XRD, ATR-FTIR and tensile stress analyses. The central composite design was utilized to optimize the water flux and dye rejection. Using CCD, the effect of the membrane thickness, PEG (pore-former), and chitosan content on the flux and rejection were studied. The optimal conditions were obtained as; 10 wt% PEG, 3 wt% chitosan, and 200 μm membrane thickness. The optimized flux and rejection were obtained 78.94 L m−2 h-1 and 90.3 %, respectively. The antifouling of the membranes was also examined by the filtration of BSA protein solution. The membranes showed comparable performance; however, extremely higher permeability (78.94 L m−2 h-1 bar -1) compared to similar TFC membranes that guarantee the economic favorability of the filtration process.

Electron Transport Improvement of Perovskite Solar Cells via a ZIF-8-Derived Porous Carbon Skeleton

To improve electron transport rate of perovskite solar cells, a ZIF-8-derived porous carbon skeleton layer is prepared by carbonizing the ZIF-8 thin film on conducting glass as the electron transport skeleton of a perovskite solar cell. Polyvinylpyrrolidone is added during the synthesis of ZIF-8 to reduce the particle size of ZIF-8 and decrease the carbonization temperature below 600 °C. The porous structure of ZIF-8 is mainly reserved at the optimized carbonization temperature. Then, TiO2 nanoparticles are deposited on the surface of a porous carbon skeleton to form an electron transport layer of a perovskite solar cell with the structure of FTO/ZIF-8-derived porous carbon layer/TiO2/perovskite/spiro-OMeTAD/Au. Because of the good conductivity of the ZIF-8-derived porous carbon skeleton, the photogenerated electron transport rate of the perovskite solar cell is increased. At the same time, the porous structure of the ZIF-8-derived carbon layer increases the contact area between the perovskite layer and t...

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.

A Comparative Study of Interdigitated Electrode and Quartz Crystal Microbalance Transduction Techniques for Metal⁻Organic Framework-Based Acetone Sensors.

We present a comparative study of two types of sensor with different transduction techniques but coated with the same sensing material to determine the effect of the transduction mechanism on the sensing performance of sensing a target analyte. For this purpose, interdigitated electrode (IDE)-based capacitors and quartz crystal microbalance (QCM)-based resonators were coated with a zeolitic–imidazolate framework (ZIF-8) metal–organic framework thin films as the sensing material and applied to the sensing of the volatile organic compound acetone. Cyclic immersion in methanolic precursor solutions technique was used for depositing the ZIF-8 thin films. The sensors were exposed to various acetone concentrations ranging from 5.3 to 26.5 vol % in N2 and characterized/compared for their sensitivity, hysteresis, long-term and short-term stability, selectivity, detection limit, and effect of temperature. Furthermore, the IDE substrates were used for resistive transduction and compared using capacitive transduction.

Metal-Organic Framework-Based Flexible Resistive Switching Memory Devices

Metal-organic framework (MOF) which consists of metal ionic nodes and organic linkers has wide applications such as hydrogen storage, catalysts, and gas sensors due to its structural and chemical versatilities. On demand for currently increasing interests for flexible and wearable electronic devices, it is suggested that some MOF materials have electrical hysteresis phenomena which can be further utilized as data storage device. The hysteresis arises from redox-active behavior of organic molecules. By selecting proper substances, realizing MOF-based resistive switching memory device could be possible. In this work, we applied zeolitic-imidazolate framework (ZIF-8), one of MOF materials, which is composed of zinc ions and imidazole molecules, as the resistive switching layer on the plastic substrate. The ZIF-8 layer on the gold-coated polyethylene terephthalate substrate is deposited by simple solution processes. Flexible ZIF-8-based resistive switching memory possesses set/reset behavior consistent over several cycles with good mechanical flexibility. The resistive switching behavior of ZIF-8-based device is thought to be due to the changes of charge conduction mechanisms from electron hopping to transporting through conjugated organic molecules. Flexible resistive switching memory device with solution-processed ZIF-8 thin film could be a promising candidate for flexible and wearable memory devices.

Thermal stability of ZIF-8 membranes for gas separations

The thermal stability of ZIF membranes is important for high temperature separation applications but has not been systematically studied. This work highlights the results of a thermal stability study of ZIF-8 membranes in terms of material structure, H2/CO2 gas permeation and separation characteristics. During binary and single gas temperature dependent permeance tests conducted from 25 to 250°C, both H2 and CO2 permeances decrease as a function of temperature. In the binary test, H2/CO2 selectivity increases between 25 and 225°C, and then decreases as temperature is further increased between 225–275°C. The results can be explained by the adsorption/diffusion mechanism. Beyond 275°C, H2/CO2 permeance and selectivity drastically increase with respect to temperature and is indicative of ZIF-8 membrane partial carbonization during the dynamic 30h temperature dependent test. The time/temperature dependency of the onset of ZIF-8 thin film structural change was deconvoluted in isothermal transient permeation experiments. Transient tests performed at 50, 100, 150 and 300°C for 24h indicate that ZIF-8 thin films maintain their crystallinity and structural integrity below 150°C. However, at temperatures of 150°C and greater the framework undergoes increased magnitudes of thermally induced carbonization as a function of temperature. Thermomechanically induced stresses between the ZIF-8 membrane thin film and α–alumina support may account for differences in static thermal stability observed when comparing ZIF-8 membranes and ZIF-8 crystalline powders.

Continuous Flow Processing of ZIF-8 Membranes on Polymeric Porous Hollow Fiber Supports for CO2 Capture.

We have utilized an environmentally friendly synthesis approach for the accelerated growth of a selective inorganic membrane on a polymeric hollow fiber support for postcombustion carbon capture. Specifically, continuous defect-free ZIF-8 thin films were grown and anchored using continuous flow synthesis on the outer surface of porous supports using water as solvent. These membranes demonstrated CO2 permeance of 22 GPU and the highest reported CO2/N2 selectivity of 52 for a continuous flow synthesized ZIF-8 membrane.

Molecular transport properties of ZIF-8 thin films in aqueous environments: The critical role of intergrain mesoporosity as diffusional pathway

We report here the first use of electrochemical tools to quantify molecular transport from aqueous solutions into ZIF-8 MOF thin films. Our experimental findings reveal that molecular transport through ZIF-8 films in aqueous environments is purely dominated by solvent-filled intergrain mesoporosity and exhibits two well-defined transport regimes.

In Situ Measurement of CO2 and H2O Adsorption by ZIF-8 Films

The adsorption of CO2 and H2O by ZIF-8 thin films was investigated using X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD) in situ under low-temperature, low-pressure conditions. Using these two techniques, we demonstrate the ability to clearly distinguish molecules that exhibit significant adsorption in the pore structure of ZIF-8, from molecules that adsorb predominantly at outer surface sites. In particular, CO2 was found to penetrate into the pore structure, while H2O resided predominantly at the surface. CO2 uptake was quantified, and mobility within the films was investigated. The ability to distinguish surface processes from those that primarily occur in the bulk is key to understanding the properties of nanoporous materials.

Facile formation of ZIF-8 thin films on ZnO nanorods

Thin films of ZIF-8 on ZnO nanorods were synthesized by casting a thin linker film followed by heating for less than an hour.

Surface and Stability Characterization of a Nanoporous ZIF-8 Thin Film

Zeolitic imidazolate frameworks (ZIFs) have been widely investigated for numerous applications including energy storage, heterogeneous catalysis, and greenhouse gas adsorption. Much of the early work has focused on the bulk properties of microcrystalline ZIFs. Herein, we focus on identifying the nature of the surface of ZIF-8 by studying a supported ZIF-8 nanoparticle film using surface characterization techniques. We have experimentally identified the presence of a zinc-rich surface terminated by carbonates and water/hydroxyl groups (in addition to the expected methylimidazole terminations) using X-ray photoelectron spectroscopy (XPS). The thermal stability of ZIF-8 thin films was also investigated using scanning electron microscopy (SEM) and temperature-programmed reaction spectroscopy (TPRS). We determined the onset of decomposition of ZIF-8 thin films to be approximately 630 K using TPRS in an ultrahigh vacuum (UHV) environment. This work presents the first characterization steps needed to study the e...

The liquid phase epitaxy method for the construction of oriented ZIF-8 thin films with controlled growth on functionalized surfaces

Highly-oriented ZIF-8 thin films with controllable thickness were grown on an -OH-functionalized Au substrate using the liquid phase epitaxy method at room temperature, as evidenced by SEM and PXRD. The adsorption-desorption properties of the resulting ZIF-8 thin film were investigated for various VOCs using the QCM technique.