Weijin Li

Metal–organic framework thin films: electrochemical fabrication techniques and corresponding applications & perspectives

Metal–organic frameworks (MOFs) hold tremendous promise for various academic and industrial applications because of their structural merits (e.g., high surface areas, enormous porosity, and regular order). Recently, processable MOF films have been sought for applications in various technologies, from separation to sensor devices. Essentially, MOFs deposited as thin films, with thicknesses ranging from nanometres to micrometres, low roughness and high homogeneity, are important for practical use and application-specific configurations, particularly in sensors and electric devices. Several innovative scientific methods have been developed for preparing MOF thin films. Among these methods, the electrochemical method is advantageous because it requires mild preparation, uses a short growth time, is easy to scale up and allows for controlling the phase/morphology and thickness by altering voltages in the process of fabricating uniform and dense MOF thin films. This review focuses on recent developments in electrochemical methods for forming metal–organic framework thin films (e.g., preparation, growth mechanisms and characterization techniques) and their corresponding applications in sensing and micro-pattern devices and identifies the challenges that must be overcome.

Electrochemical preparation of metal–organic framework films for fast detection of nitro explosives

A facile electrochemical plating method by means of applying voltage onto zinc electrodes in a 1,3,5-benzenetricarboxylic acid (H3BTC) electrolyte has been developed to prepare fluorescent MOF films (Zn3(BTC)2). The composition of as-prepared MOF films is confirmed by powder X-ray diffraction (PXRD) and the surface morphology is examined by scanning electron microscopy (SEM). Voltage and fabrication time are found to be the key parameters for the formation and morphology control of MOF films. Additionally, the as-prepared MOF films, due to their evident fluorescence, are explored for potential applications in detecting nitro explosives with a detection limit as low as 0.5 ppm. The fluorescent MOF films can be further applied to distinguish nitro explosives by varying the solution concentration. Moreover, the MOF films exhibit excellent reusability in consecutive nitro explosive detection reactions. It has been demonstrated that the electrochemical plating method reported here offers a reliable and efficient way to prepare MOF films with controllable morphology for nitro explosive detection.

In Situ Growth of Metal–Organic Framework Thin Films with Gas Sensing and Molecule Storage Properties

New porous metal-organic framework (MOF) films based on the flexible ligand 1,3,5-tris[4-(carboxyphenyl)oxamethyl]-2,4,6-trimethylbenzene (H3TBTC) were fabricated on α-Al2O3 substrates under solvent thermal conditions. The factors affecting the fabrication of films, such as the temperature of pre-activation and the dosage of the reagents, were investigated. Tuning the subtle factors on film fabrications, a series of MOF thin films with different morphologies and grain sizes were prepared. The morphology and grain size of the films are monitored by scanning electron microscopy (SEM). X-ray diffraction (XRD) and attenuated total reflection infrared (ATR-IR) were also used to characterize the MOF films. The results indicate that the temperature of pre-activation and the dosage of the reagents are the key parameters during the process of film formation. The properties of the films, especially the sensing and sorption behavior, have been studied by an optical digital cameral and ultraviolet-visible (UV-vis) spectra. The evidence shows that the films are sensitive to small organic molecules, such as methanol and pyridine. Meanwhile, the films can adsorb small dye molecules. Thus, the films may have potential applications in either organic vapor sensing or storage of small dye molecules.

Integration of metal-organic frameworks into an electrochemical dielectric thin film for electronic applications

The integration of porous metal-organic frameworks onto the surface of materials, for use as functional devices, is currently emerging as a promising approach for gas sensing and flexible displays. However, research focused on potential applications in electronic devices is in its infancy. Here we present a facile strategy by which interpenetrated, crystalline metal-organic framework films are deposited onto conductive metal-plate anodes via in situ temperature-controlled electrochemical assembly. The nanostructure of the surface as well as the thickness and uniformity of the film are well controlled. More importantly, the resulting films exhibit enhanced dielectric properties compared to traditional inorganic or organic gate dielectrics. This study demonstrates the successful implementation of the rational design of metal-organic framework thin films on conductive supports with high-performance dielectric properties.

Palladium nanoparticles in situ generated in metal–organic films for catalytic applications

Palladium nanoparticles were first in situ generated in metal–organic films for catalytic applications. Layer-by-layer assembly of metal–organic films consisting of rigid-rod chromophores connected by terminal pyridine moieties to palladium centers on solid substrates was presented. Bipyridyl and polypyridyl ligands were used as building blocks to explore the influence of different ligand structures on catalytic properties. Metal–organic films were characterized by UV-Vis spectra, atomic force microscopy (AFM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The results show that the deposition mechanism of metal–organic films is perfect layer-by-layer self-assembling with complete surface coverage and regular growth. Moreover, the catalytic activity toward the hydrogenation of olefin was investigated. Based on XPS and TEM, the catalytic activity toward the hydrogenation of olefin was ascribed to the in situ formation of Pd nanoparticles from Pd ions in metal–organic films. This film material is an active catalyst for the hydrogenation of olefin under mild conditions. Furthermore, catalytic results indicated that monodentate bipyridyl ligands exhibited superior catalytic activity than tridentate polypyridyl ligands. Catalytic activity is related to the loading amount of catalysts and permeability. More importantly, this study points toward the potential application of metal–organic films as heterogeneous catalysts with easy separation and good recyclability.

Excellent Efficacy of MOF Films for Bronze Artwork Conservation: The Key Role of HKUST-1 Film Nanocontainers in Selectively Positioning and Protecting Inhibitors

Development of metal-organic framework (MOF) films for selectively positioning inhibitors in metallic anticorrosion applications remains a substantial challenge due to the difficulty of controlling the arrangement of inhibitor molecules in MOF pores. Cetyltrimethyl ammonium bromide (CTAB), which contains hydrophobic and hydrophilic tails, was chosen as a prototypical inhibitor and was selectively located in the pores of the classic HKUST-1 thin film on a metallic surface. Experimental results reveal that the prepared CTAB@HKUST-1 film displays good metallic anticorrosion performances, especially for bronze conservation. A possible anticorrosion mechanism of CTAB@HKUST-1 is proposed and fully discussed. The study provides an avenue for developing MOF-based thin films for metallic anticorrosion applications to address the environmental development issues related to corrosion.

Bimetallic layer-by-layer films and their application in catalytic hydrogenation of olefin

Since bimetallic catalysts exhibit enhanced activity and stability over their monometallic counterparts, bimetallic film catalysts (Pd/bpy/Pt/bpy)n, were first prepared by alternating immersions of a substrate in a K2PdCl4 precursor (or K2PtCl4 precursor) and bpy (bpy = 4,4′-bipyridyl) solutions through a layer-by-layer (LbL) self-assembly method. The self-assembly of bimetallic films was characterized by UV-vis spectra. The absorbance increases with the number of bilayers and the film growth shows a good linear correlation between the optical absorption and the number of (Pd/bpy/Pt/bpy)n. Pd and Pt contents of (Pd/bpy/Pt/bpy)5 films were determined to be 2.0 × 10−7 and 8.4 × 10−8 mol using an inductively coupled plasma OES spectrometer (ICP-OES), respectively. The as-prepared (Pd/bpy/Pt/bpy)5 film demonstrates a remarkable catalytic activity toward hydrogenation of olefin bearing different characteristics under mild conditions. The relationship between the catalytic activity and the number of bilayers was investigated. The catalytic activity of the (Pd/bpy/Pt/bpy)n films increases with the number of bilayers below 4 bilayers for styrene hydrogenation and remains unchanged after 4 bilayers. X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) illustrate that Pd and Pt ions of (Pd/bpy/Pt/bpy)n films are in situ reduced into Pd and Pt nanoparticles (NPs) during the hydrogenation process, resulting in high catalytic activity. In addition, the control experiments show that the catalytic activity of our bimetallic catalysts is higher than that of the prereduced bimetallic catalyst and the physical mixtures of the Pd and Pt film catalysts. As compared to traditional heterogeneous catalysts, the film catalysts have superior advantages of easy separation and good recyclability, because they are easily removed from the reaction mixture without separation filtration.