Fabrication Of Hierarchical Porous Materials Based On Pickering Emulsion And Their Potential Applications

Hierarchical porous structures are ubiquitous in nature,especially in biological organisms,which play a very important role in the fabrication of cell,tissue and even organ.Inspired by nature,so far scientists have prepared a large number of materials with hierarchical porous structures,and made great progress in the design and synthesis of hierarchical porous materials as well as their applications in many industries.However,compared to naturally occurring analogues,manmade hierarchical porous structures need to be further improved and optimized,since these porous structures are still relatively“simple”,less robust and less functionalities.Moreover,the controllable and scalable production of hierarchically porous structures with desired properties remains a big technological challenge.In this dissertation,we have developed a multi-template method based Pickering emulsion associated with sol-gel and molecular self-assembly,successfully fabricated a series of hierarchical porous materials,and explored their application in supercapacitor.Pickering emulsion,also known as solid particles stabilized emulsion,not only is much more stable than conventional emulsions stabilised by surfactant,but can act as double tempaltes for the fabrication of hierarchical porous materials.This is a facile,versatile and scalable process,which could be used to prepare many kinds of hierarchical porous materials.There are four major sections in the dissertation,and the main research contents and conclusions are shown as follows:1.Fabrication of hierarchical porous silica capsules based on oil-in-water Pickering emulsionHierarchical silica capsules?HSC?with integrated micro-meso-macro pores have been successfully fabricated by using a facile multi-template method based on CaCO₃ rods stabilized oil-in-water emulsion.There are 5 types of different pores in the capsules,ranging from 1 nm to 100?m?spanning 5 orders of magnitude?.The micro-pores centered at 0.7 nm and 1.2 nm are from the hydrolysis and poly-condensation of TEOS and APTES;the mesopores at 2.8 nm are templated by the micelles of CTAB;the hollow tubes are templated from CaCO₃ rods with the length of around 20?m and the diameter of around 1.0?m,and the spherical cavities of around 100?m are templated from the emulsion droplets stabilized by CaCO₃ rods.Here CaCO₃ rods play a critical role for the creation of robust capsules.The micro-rods could self-assemble at the oil-water interface to form 2D polydomain nematic structure,which significantly improves the stability of emulsion and makes the fabrication of silica capsules possible.The shell of capsules is constructed by silica hollow tubes,which perfectly replicates the 2D polydomain nematic structure of CaCO₃ rods stabilized o/w emulsion droplets.Therefore,the hierarchical porous silica capsules are still robust,although their size could reach 100?m and higher.2.Fabrication of hierarchical porous silica capsules based on water-in-oil Pickering emulsionSimilarly,by using CaCO₃ rods stabilized w/o emulsions and CTAB as multi-templates,we have successfully prepared robust hierarchical porous silica capsules.The microstructures of silica capsules are similar with those prepared from the o/w system but more robust,containing micro-pores of 0.7 nm and 1.2 nm,mesopores of 2.8 nm,hollow tubes of 20?m and spherical cavities of 100?m.It is worthy to note that silica grows in different ways in both o/w and w/o emulsion systems,especially the silica layer with numbers of amino groups from APTES.It grows at the outer surface of shell in the o/w emulsion,while deposits onto the inner surface of shell in the w/o emulsion.As it is expected,the second silica layer without amino group mainly grows at the complementary outer or inner surface respectively.Therefore,to some extent,these two tyes of silica capsules have Janus shell structures,which can be tuned and modified according to the applications.So far,both o/w and o/w emulsions have been used as templates to create hierarchical porous structures,and it means this multi-template method has good flexibility and feasibility.3.One-pot synthesis of silica capsules or spheres with hierarchical structuresHierarchical silica capsules or spheres have been prepared through one-pot synthesis method based on CaCO₃ rods stabilized w/o Pickering emulsion.Here APTES plays an important role as well,and the microstructure of silica could be tuned by simply changing the APTES content.When APTES content increases from 0.04 mL to 1.20 mL,the microstructures evolves from hollow capsule to microspheres constructed by silica nanoparticles,and meanwhile the size of silica nanoparticles decreases.Thanks to the self-assembly and piling of nanoparticles,there are hierarchical porous structures in silica microspheres,and the pore size distribution is correlated with the size of silica nanoparticles and could be tuned by changing APTES content.Since there are lots of amino groups in the silica microspheres,by employing those as carriers,silica/gold composite microspheres have been fabricated with even distribution of gold nanoparticles.4.Fabrication of hierarchical porous carbon?HPC?and exploratory study of its electrochemical performanceHierarchical porous carbon?sphere?has been fabricated by employing hierarchical silica spheres as the hard template and triblock copolymer Pluronic P123 as the soft template.Three types of pores were tunably constructed in the HPC spheres in a wide size range of around 3.0to 100 nm,e.g.the ordered mesopores at 2.8 nm templated by the micelles of Pluronic P123,the mesopores ranging from 5 nm to 20 nm templated by the primary silica nanoparticles,and the mesopores/macrospores ranging from 20 nm to 100 nm templated by the aggregates of silica nanoparticles.Since the silica spheres were in situ constructed by silica nanoparticles,which were formed from the sol-gel system of TEOS and APTES,the porous structures of HPCs were simply controlled by changing the size of silica nanoparticles,i.e.by varying the APTES content.Thanks to their high surface areas?760 m²/g?and interconnected pores,the HPCs exhibited good electrochemical performance with specific capacitances of up to 170F/g and outstanding cycling stability without capacitance loss after 5000 cycles.