Metal Organic Framework Thin Films Research Articles

(Keynote) Conduction and Photoconduction in Fullerene- and Porphyrin-Containing Metal-Organic Framework Thin Films

Photoconductivity is a characteristic property of semi-conductors, enabling various applications such as photoresponsive transistors and light-sensing. Some crystalline metal-organic framework, MOF, composed of functional organic components show interesting photoconduction properties. Here, we compare the light-responsive conductivity of four different MOF thin films with similar structure, but different compositions, possessing C60 fullerene and porphyrin or phenyl moieties. The results show that the embedment of fullerene, as well-known n-conducting component, increases the conductivity but shows little photo-response. While the total conductivity values are small, the conduction in the porphyrin sample shows noticeable photo-response, in particular when exciting the Soret band with blue light. Taking advantage of donor-acceptor interactions, the combination of C60 and porphyrin in close proximity of the MOF structure, results in thin films with strong photo-response and conductivities increased by 2 orders of magnitude upon blue light irradiation.

(Keynote) Electromagnetic Field Stimulated MOFs

Metal-Organic Frameworks, consisting of inorganic nodes and organic linker molecules, are (mainly) porous materials with record holding inner surface areas, tailor-made properties, and adjustable pore windows. External stimuli on MOFs can be applied and achieve control over crystallization, gas transport properties, pore windows, and many other effects: breathing, dipolar moment, etc., which is extremely interesting for membrane science.The keynote will present an overview on novel types of applied physics to membrane separations to enhance green production and carbon capture: we can use novel MOF materials as porous liquids or MOF thin films and influence their properties with external stimuli, such as electromagnetic fields. This leads to interesting interactions: adsorption and desorption can be changed, as well as an in-situ control over the continuous gas separation on the nanometer scale can be done through membrane layers.

Current Progress and Future Directions in Gas‐Phase Metal‐Organic Framework Thin‐Film Growth

Deposition of materials as a thin film is important for various applications, such as sensors, microelectronic devices, and membranes. There have been breakthroughs in gas-phase metal-organic framework (MOF) thin-film growth, which is more applicable to micro- and nanofabrication processes and also less harmful to the environment than solvent-based methods. Three different types of gas-phase MOF thin film deposition methods have been developed using chemical vapor deposition (CVD), atomic layer deposition (ALD), and physical vapor deposition (PVD)-CVD combined techniques. The CVD-based method basically converts metal oxide layers into MOF thin films by exposing the surface to ligand vapor. The ALD-based method allows growing MOF thin films following layer-by-layer (LBL) growth by sequentially exposing gas-phase metal and ligand precursors. The PVD-CVD method uses PVD for metal deposition and CVD for ligand deposition, which is similar to LBL growth. These gas-phase growth methods can broaden the use of MOFs in diverse areas. Herein, the current progress of gas-phase MOF thin film growth is discussed and future directions suggested.

Redox-Hopping and Electrochemical Behaviors of Metal–Organic Framework Thin Films Fabricated by Various Approaches

In this study, spatially dispersed redox-active iridium sites are installed on the nodes of a zirconium-based metal–organic framework (MOF), UiO-66, utilizing a self-limiting solution-phase solvoth...

N-Heterocyclic Carbene as a Surface Platform for Assembly of Homochiral Metal-Organic Framework Thin Films in Chiral Sensing.

N-heterocyclic carbenes (NHCs) have attracted increasing attention on surface assembly due to their strong metal binding property, but an NHC-modified metal surface as a new growth platform to assemble other functional materials is still a challenge. Here, we report the preparation and chiral sensing properties of homochiral metal-organic framework thin films on carboxyl-containing NHC self-assembled monolayer-modified gold (Au(NHC)) substrates. By using a liquid-phase epitaxial layer-by-layer method, enantiopure [Cu2(cam)2dabco]n thin films with preferred [110] crystal orientation have been successfully grown on Au(NHC) surfaces. The results of electrochemical cyclic voltammetry and quartz crystal microbalance uptakes of (R)- and (S)-1-phenylethanol show that the chiral porous thin film on the robust Au(NHC) surface has good enantiomeric electrochemical recognition and enantioselective adsorption. The present work is a new step to develop metal-NHCs as surface platforms for the preparation of multifunctional thin films for sensing applications.

In Situ Spectroscopy of Calcium Fluoride Anchored Metal-Organic Framework Thin Films during Gas Sorption.

Surface‐mounted metal–organic frameworks (SURMOFs) show promising behavior for a manifold of applications. As MOF thin films are often unsuitable for conventional characterization techniques, understanding their advantageous properties over their bulk counterparts presents a great analytical challenge. In this work, we demonstrate that MOFs can be grown on calcium fluoride (CaF2) windows after proper functionalization. As CaF2 is optically (in the IR and UV/Vis range of the spectrum) transparent, this makes it possible to study SURMOFs using conventional spectroscopic tools typically used during catalysis or gas sorption. Hence, we have measured HKUST‐1 during the adsorption of CO and NO. We show that no copper oxide impurities are observed and also confirm that SURMOFs grown by a layer‐by‐layer (LbL) approach possess Cu+ species in paddlewheel confirmation, but 1.9 times less than in bulk HKUST‐1. The developed methodology paves the way for studying the interaction of any adsorbed gases with thin films, not limited to MOFs, low temperatures, or these specific probe molecules, pushing the boundaries of our current understanding of functional porous materials.

In Situ Spectroscopy of Calcium Fluoride Anchored Metal–Organic Framework Thin Films during Gas Sorption

AbstractSurface‐mounted metal–organic frameworks (SURMOFs) show promising behavior for a manifold of applications. As MOF thin films are often unsuitable for conventional characterization techniques, understanding their advantageous properties over their bulk counterparts presents a great analytical challenge. In this work, we demonstrate that MOFs can be grown on calcium fluoride (CaF2) windows after proper functionalization. As CaF2 is optically (in the IR and UV/Vis range of the spectrum) transparent, this makes it possible to study SURMOFs using conventional spectroscopic tools typically used during catalysis or gas sorption. Hence, we have measured HKUST‐1 during the adsorption of CO and NO. We show that no copper oxide impurities are observed and also confirm that SURMOFs grown by a layer‐by‐layer (LbL) approach possess Cu+ species in paddlewheel confirmation, but 1.9 times less than in bulk HKUST‐1. The developed methodology paves the way for studying the interaction of any adsorbed gases with thin films, not limited to MOFs, low temperatures, or these specific probe molecules, pushing the boundaries of our current understanding of functional porous materials.

Studying ZIF-8 SURMOF Thin Films with a Langatate Crystal Microbalance: Single-Component Gas Adsorption Isotherms Measured at Elevated Temperatures and Pressures.

Characterizing the adsorption behavior of thin films at technical operation ranges is highly relevant in order to obtain appropriate experimental parameters, as well as to evaluate their possible integration in multiple applications. Nonetheless, gathering such experimental data is not a trivial task. In the case of metal-organic framework thin films, a particular interesting class of highly porous coatings, determination of adsorption isotherms at elevated temperatures and pressures is not commonly reported. Using a custom-designed langatate crystal microbalance following the principle for piezoelectric microbalances, but under harsher conditions, allowed us to present adsorption equilibrium measurements on a zeolitic imidazolate framework-8 (ZIF-8) surface-mounted metal-organic framework (SURMOF) thin film at temperatures between 35 and 100 °C and at a pressure range up to 10 bars. The layer-by-layer liquid-phase epitaxy approach was used to deposit ZIF-8 SURMOF thin films on langatate crystals. X-ray diffraction, infrared reflection absorption spectroscopy, and scanning electron microscopy combined with energy dispersive X-ray mapping were used to gather deeper information on the ZIF-8 thin film growth. Single-component isotherms were collected for different gases of practical interest, that is, CO2, C2H6, C2H4, C3H8, and C3H6. The measurements were obtained at 35, 50, 75, and 100 °C at a gauge pressure range up to 10 bars. The collected data showed a clear preferential adsorption of ethane over ethylene, whereas only a slight difference was found between propane and propylene. Additionally, the experimental data allowed for the determination of adsorption equilibrium constants and saturation loadings of the ZIF-8 material as a thin SURMOF film, which is of great importance for its integration in multiple applications, such as sensor devices or in membrane-based separation.

Conductive Metal-Organic Framework Thin Film Hybrids by Electropolymerization of Monosubstituted Acetylenes.

1-Hexyne monomers were potentiostatically electropolymerized upon confinement in 1D channels of a surface-mounted metal-organic framework Cu(BDC) (SURMOF-2). A layer-by-layer deposition method allowed for SURMOF depostition on substrates with prepatterned electrodes, making it possible to characterize electrical conductivity in situ, i.e., without having to delaminate the conductive polymer thin film. Successful polymerization was evidenced by mass spectroscopy, and the electrical measurements demonstrated an increase of the electrical conductivity of the MOF material by 8 orders of magnitude. Extensive DFT calculations revealed that the final conductivity is limited by electron hopping between the conductive oligomers.

Highly Selective Metal-Organic Framework Textile Humidity Sensor.

The increase in demand and popularity of smart textiles brings new and innovative ideas to develop a diverse range of textile-based devices for our daily life applications. Smart textile-based sensors (TEX sensors) become attractive due to the potential to replace current solid-state sensor devices with flexible and wearable devices. We have developed a smart textile sensor for humidity detection using a metal–organic framework (MOF) as an active thin-film layer. We show for the first time the use of the Langmuir–Blodgett (LB) technique for the deposition of a MIL-96(Al) MOF thin film directly onto the fabrics containing interdigitated textile electrodes for the fabrication of a highly selective humidity sensor. The humidity sensors were made from two different types of textiles, namely, linen and cotton, with the linen-based sensor giving the best response due to better coverage of MOF. The TEX sensor showed a reproducible response after multiple cycles of measurements. After 3 weeks of storage, the sensor showed a moderate decrease in response. Moreover, TEX sensors showed a high level of selectivity for the detection of water vapors in the presence of several volatile organic compounds (VOCs). Interestingly, the selectivity is superior to some of the previously reported MOF-coated solid-state interdigitated electrode devices and textile sensors. The method herein described is generic and can be extended to other textiles and coating materials for the detection of toxic gases and vapors.

Metal-Organic Framework-Enhanced ELISA Platform for Ultrasensitive Detection of PD-L1.

The programmed cell death ligand 1 (PD-L1) protein has emerged as a predictive cancer biomarker and sensitivity to immune checkpoint blockade-based cancer immunotherapies. Current technologies for the detection of protein-based biomarkers, including the enzyme-linked immunosorbent assay (ELISA), have limitations such as low sensitivity and limit of detection (LOD) in addition to degradation of antibodies in exposure to environmental changes such as temperature and pH. To address these issues, we have proposed a metal-organic framework (MOF)-based ELISA for the detection of the PD-L1. A protective coating based on Zeolitic Imidazolate Framework 8 (ZIF-8) MOF thin film and polydopamine-polyethylenimine (PDA-PEI) was introduced on an ELISA plate for the improvement of antibody immobilization. Sensitivity and LOD of the resulting platform were compared with a conventional ELISA kit, and the bioactivity of the antibody in the proposed immunoassay was investigated in response to various pH and temperature values. The LOD and sensitivity of the MOF-based PD-L1 ELISA were 225 and 15.12 times higher, respectively, compared with those of the commercial ELISA kit. The antibody@ZIF-8/PDA-PEI was stable up to 55 °C and the pH range 5-10. The proposed platform can provide sensitive detection for target proteins, in addition to being resistant to elevated temperature and pH. The proposed MOF-based ELISA has significant potential for the clinical and diagnostic studies.

Vapor-assisted epitaxial growth of porphyrin-based MOF thin film for nonlinear optical limiting

Fabrication of metal-organic frameworks (MOFs) thin films has been an efficient way to expand their functionalities and applications. Here, we use the vapor-assisted deposition (VAD) method to epitaxially grow a porphyrin-based MOF PCN-222 film. That is, vapor source assists to deposit pre-treated precursor solution on quartz substrate to form a continuous PCN-222 film. Furthermore, utilizing the post-treated encapsulation of functional carbon-based nanoparticles, the carbon nanodots (CND) and Pt doped CND (Pt/CND) are well loaded into the pores of PCN-222 film, the size (~3.1 nm) of which is highly close to the pore size of the corresponding MOF (~3.7 nm). The Z-scan results reveal that PCN-222 film exhibits high reverse saturable absorption. In addition, encapsulation of carbon based nanodots into PCN-222 film could enhance the nonlinear optical limiting effect benefiting from the host-guest combination. This study serves to present both the available toolbox of thin film preparation and high potential for precise synthetic nanocomposite films in optical limiting devices.

Layer-by-layer assembled dual-ligand conductive MOF nano-films with modulated chemiresistive sensitivity and selectivity

In this paper, a dual-ligand design strategy is demonstrated to modulate the performance of the electronically conductive metalorganic frameworks (EC-MOFs) thin film with a spray layer-by-layer assembly method. The thin film not only can be precisely prepared in nanometer scale (20–70 nm), but also shows the pin-hole-free smooth surface. The high quality nano-film of 2,3,6,7,10,11-hexaiminotriphenylene (HITP) doped Cu-HHTP enables the precise modulation of the chemiresistive sensitivity and selectivity. Selectivity improvement over 220% were realized for benzene vs. NH3, as well as enhanced response and recovery properties. In addition, the selectivity of the EC-MOF thin film sensors toward other gases (e.g. triethylamine, methane, ethylbenzene, hydrogen, butanone, and acetone) vs. NH3 at room temperature is also discussed.

Electrodeposition of Zn-Co-Terephthalate MOF and Its Conversion to Co-Doped ZnO Thin Films

Energy security and sustainability are the most important challenges of mankind for their survival towards the 22nd century. While renewable electricity by solar and wind has already become so cheap, technologies for its conversion to easily storable chemical energies are urgently needed. Electrocatalysts not dependent on noble metals therefore need to be developed.We have recently succeeded in microwave (MW)-assisted hydrothermal synthesis of Co-doped ZnO nanoparticles, which exhibit catalytic activity towards water oxidation superior to Co3O4, although the electrode prepared by paste coating faced problems of its stability due to imperfect connection of the particles [1]. Electrodeposition can overcome this problem since well crystallized, stable and perfectly conducting ZnO thin films can be obtained. However, it is not at all straightforward to dope cobalt because of the large different of solubility (K sp = 5.92 × 10-15 and 3 × 10-17 for Co(OH)2 and Zn(OH)2, respectively), so that almost pure ZnO is solely electrodeposited even in the presence of Co2+ ions in the bath.Taking advantage of structural flexibility of metal-organic framework (MOF) could be a possible solution to achieve co-precipitation of Zn and Co. We have recently achieved cathodic electrodeposition of Zn-terephthalate (TPA) MOF thin films by addition a TPA into the ZnO electrodeposition bath [2]. Layered MOF structure was self-assembled during precipitation by bridging of double-layered Zn hydroxide with TPA.Herein, we report successful electrodeposition Zn-Co-TPA MOF precursor thin films and its conversion to oxides to yield Co-doped ZnO. Compositional and structural characterization as well as their electrocatalytic activities for oxygen evolution reaction (OER) are discussed.Potentiostatic electrolysis at an F-doped SnO2 (FTO) coated glass rotating disk electrode (RDE, ω = 500 rpm) was carried out at -1.0 V (vs. Ag/AgCl) in an O2-saturated aqueous electrolyte (70ºC) containing 5 mM ZnCl2, 200 μM TPA, and 0,5 mM CoCl2, for 30min. The film was annealed at 450℃ for 1 hour, under air.A white film electrodeposited in the presence of CoCl2 turned into green film after annealing, as expected for the formation of Co-doped ZnO. (Fig. 1a). X-ray diffraction pattern (XRD, Fig. 1b) indicates electrodeposition of Zn-TPA MOF and their conversion of ZnO after annealing both in the presence and absence of Co. The absorption spectra of the films on the other hand show clear difference between the samples, for which the one with Co and after annealing exhibit clear red-shift of onset for bandgap absorption and characteristic triple peaks at about 560, 610 and 650 nm arising from d-d transition of doped Co(II) ions (Fig. 1c). These features were commonly found for the Co-doped ZnO nanoparticles by MW reaction, so that similar or superior OER catalysis can be expected for the sample prepared by electrodeposition in this study. Figure 1

Liquid-Phase Quasi-Epitaxial Growth of Highly Stable, Monolithic UiO-66-NH2 MOF thin Films on Solid Substrates.

Invited for this month's cover is the group of Dr. Tawheed Hashem from the Karlsruhe Institute of Technology. The cover picture shows a successful synthesis of high quality, monolithic UiO-66-NH2 MOF thin films on diverse solid substrates via a low-temperature liquid phase epitaxy method. The achievement of continuous MOF-coatings with low defect densities and pronounced stability against high temperatures and hot water was proven. The new type of coatings clearly outperforms other reported types of MOF thin films. Read the full text of their Communication at https://doi.org/10.1002/open.201900324.

Monitoring of Technetium in Groundwater:Development of a Novel Qcn Based Sensor

99Tc, a pure β-emitter with Emax =294 keV, is one of the most significant (t1/2 =2.1×105 y) nuclear waste isotopes, produced by the fission of U with a yield of 6%. Under normal environmental conditions, it is mostly encountered as the pertechnetate ion, TcO4 -, which is highly soluble and consequently mobile in water. For this reason, and because it often is the first contaminant to be present in detectable levels, its monitoring into groundwater is a statutory requirement for every nuclear license site. Because of its relatively low concentration in environmental samples, current determination methods of 99Tc involve several steps, such as chemical separation from the matrix, purification and source preparation, prior to radiometric (e.g. liquid scintillation counting) or mass spectroscopic (e.g. inductively coupled plasma mass spectroscopy) determination. The detection of 99Tc may take up to several days, thus making these techniques inappropriate for an emergency situation. Hence, We report on the development of a sensor for the real time monitoring of 99Tc in groundwater based on the Quartz Crystal Nanobalance (QCN). The QCN is a piezoelectric resonator, which oscillates at a resonant frequency fs when an electric potential is applied across its body. It is capable of measuring very small changes in mass at its surface, through the change of the resonant frequency. We modify it to respond exclusively to the presence of TcO4 -, by application of a novel Ag-4,4bipyridine metal-organic framework thin film built layer-by-layer on a self-assembled monolayer of 4-mercaptopyridine on the gold surface. The film’s structure is characterised by several techniques such as XPS, AFM and XRD and we evaluate its response and selectivity by monitoring the changes in the resonant frequency as a function of the concentration of ReO4 -, which is a non-radioactive chemical surrogate for TcO4 -.

(Invited) From Chromophores to Photoactive Materials: The SURMOF Approach

Realizing molecular “Designer Solids” by programmed assembly of building units taken from libraries is a very appealing objective. Recently, metal-organic frameworks (MOFs) have attracted a huge interest in this context. Here, we will focus on the integration of chromophores into MOFs, as well as on several applications based on these highly versatile crystalline materials. For numerous MOF-based applications, in particular in the area of optics and photonics, the conventional solvothermal synthesis yielding MOF powders is not well suited. In the context of optical applications, a severe drawback is that the powder particles cause strong scattering, which makes a reliable determination of photophysical parameters difficult. To overcome these problems, we have developed a layer-by-layer (lbl) deposition method to produce well-defined, highly oriented and monolithic MOF thin films on a number of different substrates. The resulting films are referred to as SURMOFs [1,2]. The fabrication of hetero-multilayers (see Fig. 1) is rather straightforward with this lbl method. In this talk, we will describe the principles of SURMOF fabrication as well as the results of systematic investigations of electrical and photophysical properties exhibited by empty MOFs/SURMOFs [3] and after loading their pores with functional guests. Subsequently we will focus on employing SURMOFs for anisotropic exciton transport. Finally, we will point out that MOFs and SURMOFs are expected to play an important role in the digitalization of materials research.

Electrodeposition of pore-confined cobalt in metal–organic framework thin films toward electrochemical H2O2 detection

In this study, cobalt is electrodeposited within solvothermally grown thin films of a zirconium-based metal–organic framework, MOF-808, to obtain electrochemically active cobalt confined in the pore of MOF-808 crystals. With the help of the highly porous but electrically insulating MOF-808 thin film and a step-wise electrodepositing technique, cobalt can be mainly deposited within the pore of MOF-808 crystals. The resulting Co@MOF-808 thin film is applied for electrochemical H2O2 sensor in aqueous electrolytes, and its electrocatalytic activity toward H2O2 significantly outperforms that of the electrodeposited cobalt on a bare substrate. The electrochemical behavior of Co@MOF-808 thin films and the corresponding electrocatalytic mechanism toward H2O2 are investigated. The material is further used for amperometric H2O2 detection, and a sensitivity, limit of detection, and linear range of 382.27 μA/mM-cm2, 1.3 μM, and 10–450 μM can be achieved, respectively.

Methanol Oxidation Catalyzed by Copper Nanoclusters Incorporated in Vacuum-Deposited HKUST-1 Thin Films

Metal-organic framework (MOF) thin films allow the unique properties of MOFs to be incorporated into membranes and micro-electronic devices and also permit studies of catalysis employing powerful u...

Metal–Organic Framework Thin Films: Fabrication, Modification, and Patterning

Metal–organic frameworks (MOFs) have been of great interest for their outstanding properties, such as large surface area, low density, tunable pore size and functionality, excellent structural flexibility, and good chemical stability. A significant advancement in the preparation of MOF thin films according to the needs of a variety of applications has been achieved in the past decades. Yet there is still high demand in advancing the understanding of the processes to realize more scalable, controllable, and greener synthesis. This review provides a summary of the current progress on the manufacturing of MOF thin films, including the various thin-film deposition processes, the approaches to modify the MOF structure and pore functionality, and the means to prepare patterned MOF thin films. The suitability of different synthesis techniques under various processing environments is analyzed. Finally, we discuss opportunities for future development in the manufacturing of MOF thin films.