Fabrication And Properties Of Metal-organic Frameworks Thin Films With 2D Ordered Structure By Two Dimensional Colloidal Crystals Templates

As a new class of porous crystalline multi-purpose materials, metal-organic frameworks (MOFs) can be applied in many fields, such as gas storage and separation, drug delivery, heterogeneous catalysis and sensing, due to their large surface area, adjustab lepore-size on the molecular level, good chemical and thermal stability. Colloidal crystals, especially, two-dimensional colloidal crystals (i.e. monolayer colloidal crystals, MCC) have several advantages such as low preparation cost, good repeatability and unique optical properties, etc. Therefore, new functional materials with combination of the advantages of the two materials have attracted great research interests in recent years. However, the preparation of optical sensing and multifunctional superstructureis still a challenge. Herein, we made numerous efforts on the preparation of ultrathin MOF-coated monolayer colloidal crystals, multifunctional superstructures as well as MOF hollow nanoshell arrays. In addition, the formation mechanism, the optical sensing properties, separation performance of the MOF superstructures, as well as the relationship between the microstructure and properties of the products were studied in detail.1. Facile fabrication of ultrathin MOF-coated MCC for highly efficient vapor sensingThe sorption properties and structural versatility of metal-organic frameworks (MOFs) make them superior chemical sensing materials with both high sensitivity and selectivity, but the fabrication of MOF sensors with optimized performances still remains a major challenge. Herein, we propose a simple yet powerful optical sensing motif based on ultrathin MOF-coated monolayer colloidal crystals, which allows for high efficiency in vapor sensing through changes in their effective refractive index. Two optical modes exist in this sensor, namely, photonic eigenmodes and Fabry-Perot oscillations, both of which can be used as the signal transducer. Selective response to a series of alcohols, water, and acetonitrile was exhibited, reflecting well the characteristic sorption properties of the integrated MOF, with which colorimetric reporting was readily achieved. Linear response to a broad dynamic range of vapor concentration was realized. The sensitivity was found to depend closely on the thickness of the MOF coating and can be further enhanced accordingly. Ultrafast response time (<5s) and excellent recyclability were also demonstrated. These substantial improvements in performance are attributed to the efficacy of signal transduction and the enhanced pore accessibility and diffusion efficiency, which are intrinsically endowed by the optical motif design. Our findings should provide new insights into the design and fabrication of MOF sensors toward real-world applications.2. One-step asymmetric growth of continuous MOF thin films on MCC:a facile approach towards multifunctional superstructuresWe represent a facile approach towards multifunctional MOF superstructures by asymmetric growth of continuous MOFs thin films on MCC anchored at the air-solution interfaces, with which the control over spatial configuration, structural hierarchy, and overall dimensionality of MOF superstructures can be realized all at once. This interfacial growth method also endows MOF superstructures with an unprecedented transferability, which greatly facilitates the interfacing of MOF materials with other functional surfaces. We have demonstrated this by the construction of layered structures (including hybrid ones) that are promising for device applications. Taking advantage of the resulting periodic and hierarchical porous structures, the as-grown MOF superstructures lend themselves for efficient vapor sensing, size-screening of nanoparticles, and removal of dye molecules from aqueous solutions, and have exhibited a superior performance as compared to its unstructured counterpart. Therefore, this work dose not only present an efficient route in well-organizing MOF nanocrystals at the meso/macroscopie scale but also provides an inspiring example of enriching the material performance of MOFs by shaping their physical forms.3. Synthesis of MOF hollow nanoshell arrays by MCC templates and their sensing propertiesUsing the O2 plasma etched non-close-packed MCC as template, the large area of MOF thin films of the MOF hollow nanoshell arrays were prepared by using in situ growth method. Enhanced structural color saturation could be obtained by coating Pt on the surface from the theory of conjugate twin-phase modulation (CTPM) and plasmonic broadband absorber theory (PBA). Hollow nanshell arrays not only gather the quick response of ultra-thin porous MOF thin film sensor and vivid color characteristic based on the theory of CTPM-PBA at the microscopic scale, but also have the optical properties of MCC at the macroscopic scale. The sensor can detect the target molecules from the shift of the reflection peak, and the colorimetric report can be conveniently obtained via distinct color differences at the same time. This work enriches the application of MOF sensor towards the volatile organic compounds owing to its wide concentration range of detection, high selectivity, quick response and excellent reversibility.