Template-Directed Synthesis Of Metal-Organic Framework Nanofibers And Their Derivatives For Enhanced Applications

Metal-organic frameworks (MOFs) are a novel class of porous solid, which is consist of metal ions or metal clusters and multifunctional organic ligands. MOFs have shown wide applications in gas storage, CO2 capture, molecule separation, catalysis, drug or other materials support due to their porosity, high surface area, tailorability, abundant unsaturated sites, etc. Recently, MOF-derived porous carbon and metal oxide materials have attracted tremendous attention for energy storage and conversion applications owing to porous and ordered structure of MOFs. To date, many studies in this research field are focused on MOF micro-or nanocrystals, and it lacks effective control of the resulted morphology and pore texture of their derivatives. Meanwhile, it is difficult to control synthesis of the hierarchically structured MOF architectures based on the assembly of MOF nanoparticles.Based on the continued research in template-directed organization of nanoparticles in our lab, the present dissertation will focus on nanowire-directed templating synthesis of one-dimensional MOFs nanofibers and their complex, and even MOF nanofibers-based aerogels were successfully prepared. Here, we discuss the enhanced electrochemical performance of their derived materials with novel one-dimensional structure. This strategy provide a new route for rational design and synthesis of one-dimensional non-precious metal porous electrocatalysts and electrode materials with high activity. On the other hand, diverse noble metal@MOFs nanofibers were synthesized by element replacement reaction, and further discuss their organic catalytic activity based on the size selectivity and gas molecule enrichment. The main results can be summarized as follows:1. A nanowire-directed templating method was developed for preparation of high-quality MOF nanofibers. Ultrathin tellurium nanowires (TeNWs) with excellent dispersivity was employed as template to induce growth and assembly of ZIF-8 nanocrystals (one typical MOF), resulting in the formation of uniform ZIF-8 nanofibers. Further calcination, these ZIF-8 nanofibers as carbon source can be conveniently converted into nanofibrous doped porous carbon, which exhibit high surface area, hierarchical pores and complex network structure. Compared with bulk carbon by direct carbonization of MOF crystals, the as-obtained nanofibrous doped carbon exhibit better electrocatalytic performance. Further doped by phosphorus (P) species, the co-doped carbon nanofibers exhibit high ORR activity with half-wave potential of ～-0.161 V, which is even better than the benchmark of the commercial Pt/C catalyst. This synthetic route will offer new opportunities for rational design and synthesis of diverse MOF nanofibers and their derived porous carbon or metal oxide materials.2. A new nanofibers-based MOF aerogel was synthesized by ice freeze-drying technology. Such aerogel as perfect model can effectively reduce contact among MOF nanofibers in high temperature calcination, resulting well-dispersed hollow porous carbon nanofibers. The final products can be as electrocatalyst and electrode material for enhanced their electrochemical performance. Furthermore, well-dispersed carbon nanofibers was introduced into sulfur support for the encapsulation of sulfur into their hierarchical micro- and mesopores as well as hollow nanostructure. The final composite exhibited excellent electrochemical features, good cycling stability and high coulombic efficiency as the cathode electrode materials for lithium-sulfur batteries. Meanwhile, an ultrahigh ORR active catalyst with half-wave potential of ～-0.133 V was prepared by carbonized a bimetal MOF nanofibers-based aerogel.3. Carbonaceous nanofibers (CNF)-based MOF composite nanofibers and aerogel were prepared by step-by-step route. Three classical MOF materials (HKUST-1, ZIF-8, MIL-100-Fe) were selected as research model for synthesis of CNF@MOF composite nanofibers and gel, and their gas sorption capacity were studied. CNF@ MIL-100-Fe was further converted into well-dispersed Fe2O3 nanotube by pyrolysis in air, and further investigated their performance in Li-ion battery.4. Several noble metal nanowire or nanotube and MOF nanofibers composite were designed and synthesis. Based on element replacement reaction from tellurium to palladium and platinum, these noble metal or alloy@UIO-66 and UIO-66-NH2 nanofibers were obtained and further discussed their organic catalytic activity based on the small molecule selectivity and gas enrichment and photocatalytic activity based on one-dimensional co-catalyst.5. A facile and general polydimethysiloxane (PDMS)-coating technology to enhance moisture/water stability of metal-organic frameworks has been successfully developed. Here, we employed three vulnerable MOFs (MOF-5, HKUST-1 and [Zn(bdc)(ted)0.5]·2DMF·0.2H2O) to investigate their water stability before and after PDMS coating. The surface of these MOFs were successfully coated by PDMS, which form a hydrophobic protective layer. Upon moisture/water treatment, the porosity of PDMS-coated MOFs exhibits nearly 100% retained while the pristine MOFs lose their porosity completely. Compared with other post-modified methods, such PDMS-coating approach is not only facile but also shows significant superiority regarding porosity preservation. Remarkably, the PDMS-coated MOF have well retained porosity and surface area, as well as the active sites that remain accessible to substrates after water treatment.