Microwave Synthesis Of Composite Nanomaterials And Their Properties In Environmental Energy

Environmental pollution and energy crisis are the most important global challenges,which damage sustainable development of society and economy,the human health,the national security and biological balance.Nanomaterials,especially those with low-dimensional nanostructures,exhibit excellent strength in both environmental and energy fields due to their suitable pore structure,large specific surface area and directional electron-transfer pathways.The heterojuntion structures from low-dimensional nanocomposites have been widely used in catalysis,energy and environment,which have become hot topics in nanomaterials research.At present,the preparation of low-dimensional nanocomposites generally utilizes traditional heating process with heat transferring from the outside to the inside.This process often exists less uniform temperature,high energy consumption,time waste and other shortcomings,together with a lot of side effects,which is not suitable for precisely controlling the interface and morphology of the composites.Meanwhile,it is also less conducive to design and optimize the new specific structures for catalysis or energy storage.To solve these problems,microwave chemical synthesis method has been developed using the thermal effect by carbon materials or nano-sized metals under microwave irradiation.New concepts of microwave dipole nano-antennas and microwave-induced super-hot surface effects are proposed.Different catalytic and energy-storage nanocrystals materials assembled on the surface of carbon nanotubes（CNTs）,graphene or copper nanowires（CuNWs）have been designed synthesized.Besides the advantages of easy control,environment-friendly,fast and low energy consumption,the present strategy could be also applied to the structural regulation and in-situ modification simultaneously.It could provide a new fabrication technology for synthesizing advanced functional nanomaterials.The main content and innovation of this thesis are divided into five parts:（1）We have developed a general microwave-antenna strategy for in situ synthesizing threaded nanocomposites.The 1-dimensional materials like CNTs with strong ability for absorbing microwave were used as microwave antennas to generate the high-temperature local surface “super-hot” dots.Such “super-hot” dots would induce adsorption,hydrolysis or coordination of metallic ions,followed by nucleation,crystal-growth and self-assembly into a unique architecture with metal oxides andmetal organic frameworks（MOFs）nanocrystals threaded by CNTs.In CNT-TiO₂ nanocomposites,the CNTs provided electron conductive pathways facilitating better photocatalytic transformation and energy storage.We believe this new microwaveantenna strategy can be extended to in situ fabricating other 1-dimensional（1D）binary or ternary nanocomposites and even 2-,or 3-dimensional（2D,3D）materials threaded nanocomposites with controllable architectures like those found in nature,which may offer more opportunities for their applications in environmental and energy fields.（2）Under the concept of microwave dipole nano-antenna,we have developed porous CuO nanotubes/graphene hybrids with sandwich-like nano-architecture for high-performance anodes in lithium-ion batteries.By a simple microwave-assisted insitu hydrothermal and a subsequent annealing approach,an interlaced sandwich-like framework can be constructed.With this unique nano-architecture,the as-prepared CuO-Gr electrode possesses a high specific capacity,good rate-capability and longcycling stability.The enhanced energy storage performance benefits from the synergistic effects of each component.This strategy can be further extended to construct other 1D/graphene or 1D/2D systems for their applications in energy storage,catalysis and electronic devices.（3）We have studied the effect on nano-metal with microwave irradiation,and developed a general microwave induced super-hot surface for decorating CuNWs with highly crystalline NRs or NPs.As an example,the prepared TiO₂/CuNWs exhibited a remarkably high AQY compared to noble-metal free TiO₂ photocatalysts for photocatalytic H2 generation under UV light irradiation.The direct growth of TiO₂ nanorods onto CuNWs may increase the contact interface,enhance light harvesting by multiple reflections and facilitate photoelectron transfer and enrichment,which further reduces photoelectron-hole recombination and promotes H+ reduction to H2.This study demonstrates the use of inexpensive copper nanowires as a substitute for noble metals in enhanced solar photocatalytic H2 generation and provides a general microwave-assisted solution strategy for fabricating other cascaded nanohybrids,such as NR-（NP-）decorated carbon fibers,to prepare new functional nanohybrids with fascinating properties.（4）We have applied the microwave induced super-hot surface effect for in situ synthesis of copper nanowires/ZnS（CuNWs/ZnS）hybrids with core-shell structure.During the synthesis,a small portion of Cu+ can diffuse and enter into ZnS latticewhich narrowed the band gap of ZnS,further induced visible-light response.The strong interface between CuNWs and ZnS created by microwave form a close Schottky contact which promoted the orient electron transfer.Thus,the as-prepared hybrids exhibited an enhanced activity and stability in visible light photocatalytic H2 evolution.The corresponding H2 evolution rate reached to 10722 μmol h-1 g-1 with apparent quantum efficiency（QE）of 69 % under 420 nm LED irradiation.This article not only reported a composite material with remarkably high QE among the noble-metal free visible light driven photocatalysts but also develop an advanced microwave technology for building strong interface between metal and semiconductors（5）Based on the super-hot surface effect of nano-metals,we introduce hydrogen as the reducing species into the microwave system to design a novel gas-solid interfacial reduction reaction in order to modify nano-materials.As an example,Platinum（Pt）has been used as the nano-metal which not only create super-hot surface under microwave irradiation but also provide an active site for H2 overflow.Thus the surface of Pt-TiO₂ can be reduced with the formation of Ti3+ and oxygen vacancies.The gas-solid phase reduction can inhibit Pt nanoparticles from agglomerating and the structure of the TiO₂ can be remained during the synthesis.Therefore,the prepared materials exhibit excellent activity and stability in both ultraviolet and visible light photocatalytic hydrogen production.This work not only prepared a new type of photocatalyst with high hydrogen production activity,but also designed and reported a non-destructive surface reduction method,which provided a new idea for the design of reaction path and surface modification.