Strong Magnetic Field And High Temperature Assisted Growth Of One-Dimensional Carbon Nanomaterials:Controllable Preparations,Mechanisms And Applications

Since the discovery of carbon nanotubes(CNTs)by S.Iijima in 1991,one-dimensional carbon nanomaterials,including single-walled carbon nanotubes(SWCNTs),multiwalled carbon nanotubes(MWCNTs),and carbon nanofibers(CNFs),have been extensively studied in the world due to their excellent and unique properties and great potential applications.For example,compared with silicon,the conductivity and Young's modulus of CNTs are up to 107 S/m and 1 TPa,which make them the ideal conductive channel materials for thin film transistors(TFTs).However,the application of SWCNTs in TFTs faces a challenge,namely,the controllable preparation of semiconductor SWCNTs with high-efficiency,non-destruction,non-posttreatment,large-area and high-purity deposition on any substrate.In general,the preparation of one-dimensional carbon nanomaterials with controlled morphology and microstructure lay a foundation for their applications.The controllable preparation of one-dimensional carbon nanomaterials includes:purity control,alignment control,length control,diameter control,position control,chirality control,etc.For example,by introducing weak oxidants to achieve purity control;by using template method to achieve alignment control;by using multi-step growth method to achieve length control;by using magnetic field to achieve position control;by introducing electric field to achieve the crystallinity control;by regulating the size of electrodeposited two-dimensional film of nanocrystals to achieve diameter control;by preparing specific crystal surface catalyst to achieve chiral control.However,it is a permanent task for researchers to propose new methods and mechanisms to achieve the controllable preparation of one-dimensional carbon nanomaterials with high-purity,large-scale production,and application in new fields.Based on the fully understanding of one-dimensional carbon nanomaterials and the international research trend,we conducted systematic studies on the one-dimensional carbon nanomaterials,including controllable preparation,growth mechanism,performance,and application.We proposed a new method to prepare high percentage semiconducting single-walled carbon nanotubes(s-SWCNTs)by using strong magnetic field.We developed a method for preparing the bio-inspired spider-like SWCNTs(BISS-SWCNTs)film with high strength and high conductivity by using strong magnetic field.We prepared BIS S-SWCNTs fiber with high-strength and high-conductivity by derectly using BISS-SWCNTs film as precursor and assembled a high-performance all-solid-state supercapacitor.Inspired by the high sensitivity of spider leg,we fabricated a BISS-SWCNTs/Au composite film flexible sensor with capability of detecting strain and temperature simultaneously.We systematically investigated the effect of strong magnetic field on the morphology and microstructure of CNFs.For the application of CNFs,we conducted the experiments on diamond transformation,photocatalysis and other fields.This dissertation includes ten chapters.The first chapter is introduction,including the significance and importance of the subject,and the classification,structure,and conventional preparation method of one-dimensional carbon nanomaterials.Then,the research trend and prospect of controllable preparation and application of one-dimensional carbon nanomaterials were provided.The experimental materials and methods,characterization methods and test equipment in this research were described in the second chapter.The second chapter introduces the preparation and characterization of one-dimensional carbon nanomaterials and their composites.It includes:Preparation of high percentage semiconducting SWCNTs by using strong magnetic field-assisted floating catalyst chemical vapor deposition(FC-CVD)method;Fabrication of transistors based on semiconducting SWCNTs and characterization for their electrical properties;Fabrication of BISS-SWCNTs film by using the strong magnetic field-assisted FC-CVD method;Spinning of BISS-SWCNTs fibers by using BISS-SWCNTs films as precursor and the fabrication of all-solid-state supercapacitor;Preparation of sandwich structural BISS-SWCNTs/PMMA/PMMA composite films;Fabrication of the BISS-SWCNTs/Au composite film multifunctional sensor;Preparation of CNFs with high orientation and high crystallinity by using strong magnetic field-assisted CVD method;Conversion of CNFs into diamonds in the SPS system;Synthesis of ZnO/CNF s/NiO ternary heterostructural composite photocatalytic materials.In the third chapter,a novel magnetic field-assisted FC-CVD system for controllable separation of s-SWCNTs and m-SWCNTs was designed.In this chapter,the morphology and microstructure of the s-SWCNTs were characterized in detail.The relationship between the percentage of the s-SWCNTs and the magnetic field was investigated by using Raman spectroscopy and UV-Vis-NIR absorption spectroscopy.By using this strategy,s-SWCNTs with a purity of 99%could be obtained,which was enough to construct high-performance transistors with a mobility of 230 cm2/(V s)and an on/off ratio of 104.We also established a model to quantitatively calculate the percentage of s-SWCNTs and this model showed a good match with the experimental data.Natural spider silk and spider web possess high mechanical properties due to their special structure.In the fourth chapter,bio-inspired spider silk single-walled carbon nanotubes(BISS-SWCNTs)was prepared by using magnetic field and their mechanical and electrical properties were characterized.In this chapter,inspired by the special structure of natural spider silk,we prepared the BISS-SWCNTs films that similar to the natural spider silk from the internal structure to the external morphology,which further greatly improved the mechanical properties of the SWCNTs.In order to study their mechanical properties,we investigated the stretching of BISS-SWCNTs films "in-situ" by using SEM observation.We also studied the mechanical enhancement of BISS-SWCNTs in polymer composites.The experimental results showed that with only 2.1 wt%BISS-SWCNTs added,the tensile strength and Young's modulus of the BISS-SWCNTs/PMMA composites could be improved by 300%.More importantly,the BISS-SWCNTs also retained the high conductivity and transmittance of the pristine SWCNTs film.The fifth chapter is based on the fourth chapter.A macroscopic BISS-SWCNTs fiber was fabricated and its application in all-solid-state supercapacitor was explored.This BISS-SWCNTs fiber was prepared by using BISS-SWCNTs films as precursor.Because the BISS-SWCNTs fiber possessed the spider silk-inspired structure that formed a strong interface coupling between neighboring SWCNTs,it could translate the remarkable mechanical and electrical properties of SWCNTs to the macroscopic fiber.The "tension stiffening effect" that similar to the mechanical properties of spider silk was found in BISS-SWCNTs fibers.In order to explore the mechanism of the "tension stiffening effect",we conducted the SEM observation and "in-situ"Raman measurements.We deduced that the mechanism of the "tension stiffening effect" was the stretch-induced load transfer from the network deformation to the axial extension of SWCNT.We also applied the BISS-SWCNTs fiber in all-solid-state supercapacitor,which presented a high volumetric capacitance of 128 F/cm3,revealing its excellent electrochemical performance.Based on the research of BISS-SWCNTs film in the fourth chapter,the sixth chapter introduces a novel flexible sensor with high sensitivity,long life and multifunction.Based on the special structure of high sensitivity spider leg,the BISS-SWCNTs/Au composite film was prepared by using buckled BISS-SWCNTs film as the conducting network and a crack-shaped Au film as the sensitive transducer.In this chapter,the sensitivity,reaction time,cycle performance and other parameters of the sensor were systematically tested.At the same time,the mechanism of the sensor was investigated by "in-situ" SEM observation and theoretical calculation.The results showed that the strain sensitivity was obtained from the electrical resistance variation during the Au film deformation,which caused the opening/closure of the Au nanofilm microcracks.The temperature sensitivity was caused by the expansion/shrinkage of the substrate due to the variation of temperature.Furthermore,the buckled BISS-SWNTs film,which formed a stretchable conducting network with enhanced mechanical robustness,made the sensor with high cycle performance.Finally,we also fabricated a wearable device for monitoring various human physiological signals,such as pulse,respiration,joint activity,body temperature,etc.In the seventh chapter,the effect of strong magnetic field and high temperature on the morphology and microstructure of "solid cored" CNFs was investigated.The growth of CNFs under different magnetic fields were characterized by using SEM,HRTEM and Raman spectroscopy.The results showed that under high temperature,the magnetic field can not only control the growth direction(along the direction of the magnetic field),but also reduced and homogenized the growth of CNFs.With the increase of the magnetic field,the microstructure of CNFs was gradually changed,i.e.,the strong magnetic field made the disordered "solid-cored" CNFs transform into a kind of bamboo-liked CNTs.We proposed a mechanism that the reason for these variations and transformation was due to diamagnetic property of carbon atoms,so that it had direction selectivity in the precipitation process.In the eighth chapter,a new route was found to synthesize diamond by converting "solid" CNFs with a Spark Plasma Sintering(SPS)system under low temperature and atmospheric pressure.Well-crystallized diamond crystals were obtained at the tips of the CNFs after sintering at 1500 ? and atmospheric pressure.Combining with SEM,TEM,EELS and Raman spectroscopy observations,we proposed the conversion mechanism as follows:the disorder "solid" CNFs?well crystallined CNFs?bent graphitic sheets?onion-liked rings?diamond single crystal?the bigger congregated diamond.In the SPS process,the pulsed plasma plays a crucial role in the synthesis of diamond under low-temperature and low-pressure condition.In the ninth chapter,a facile two-step CVD route was introduced as a straightforward protocol for preparing a 3D ZnO/CNF/NiO heterostructural composite.The morphology,microstructure and growth mechanism of ZnO/CNFs/NiO composites were characterized.The experimental results revealed this composite an excellent photocatalytic performance 2.5 times higher than that of regular ZnO/NiO composite.This was because of the following advantages:the 3D reticulated structure provided more nucleation sites for ZnO nanorods,in which the light would be reflected multiply and thus increased the absorption efficiency of the light;because CNFs formed two strong and compact hetero-interfaces with both ZnO nanorods and NiO substrate,respectively,it provided barrier-free access to transport photo-induced carriers(electrons and holes)between ZnO and NiO as bridge during photocatalytic process,which therefore greatly improved the separation efficiency of the electrons and holes.Chapter ten is a full summary.Finally,a brief introduction of published papers,participated projects,honors and awards,curriculum vitae and acknowledgements in the graduate were given.