Metal-Organic Framework (MOF)-Based Materials: Aerosol Synthesis and Photocatalytic Applications

Abstract

Metal-organic frameworks (MOFs) have been attracting great attention in the past several decades mainly because of their amazing properties, including tunable surface chemistry, flexible structure, large surface area, and huge porosity. Endorsed by those merits, MOFs have been applied in a wide range of applications, such as catalysis, gas separation, drug delivery, and sensing. Typically, MOFs are synthesized via the hydrothermal method, which, however, is difficult to scale up and requires long reaction durations (e.g., from hours to days). To achieve the full potentials of MOFs, the exploration of a novel strategy is necessary for the facile and fast synthesis of MOFs. Here in this dissertation, the aerosol route was presented as a facile route to synthesize MOFs and MOF-based composites. The aerosol route not only enabled fast crystallization of MOFs (i.e., within seconds), but also allowed continuous tuning of MOF’s properties by simply adjusting the operating parameters (e.g., temperature, pressure, and precursor conditions). To map out the formation mechanism of MOFs inside the microdroplets, systematic experimental and simulation work were carried out, which demonstrated that the fast heat and mass transfer during the aerosol route played a vital role in the rapid synthesis of MOFs. Beyond the synthesis of MOFs, the photocatalytic applications of MOF-based materials for energy and environmental sustainability were also studied in detail. More specifically, several efficient MOF-based composite photocatalysts were designed, including HKUST-1/TiO2, HKUST-1/TiO2/Cu2O, ZIF-8/ZnO, and MIL-100(Fe)/TiO2. The composite photocatalysts exhibited remarkable efficiencies towards either CO2 photoreduction or water remediation. In-depth exploration of the photocatalytic mechanism was carried out with the aid of several advanced techniques, such as in situ diffuse reflectance infrared Fourier spectroscopy (DRIFTS), photoluminescence spectroscopy, grazing-incidence wide-angle X-ray scattering, and ultrafast transient absorption spectroscopy. Meanwhile, the density functional theory (DFT) calculation was also applied to provide further mechanistic insights. The results demonstrated that MOFs acted as excellent co-catalysts, which not only facilitated molecule adsorption and activation, but also promoted the separation of the photo-induced charge carriers, leading to increased charge carrier densities in the photocatalytic systems for significantly enhanced efficiencies. The work from this dissertation is expected to broaden the synthesis strategies for the synthesis of MOF-based materials and advance the fundamental understanding of MOFs’ roles in photocatalytic applications, which should have a great impact on the rational design of MOF-based composite photocatalysts for energy and environmental sustainability.