Synthesis,Growth And Regulation Of Nanostructures By Electron Beam And Laser Beam

Electron beam and laser beam are two typical energy-efficient,non-polluting energy-carried beams.Both of them can induce chemical reactions and synthesize or regulate nanomaterials.In-depth study on the material synthesis under the two energy-carried beams is conducive to the developing new synthetic processes,obtaining unique material morphology and structure,and ultimately improving the physical and chemical properties.In this thesis,the regulation mechanism of electron beam and laser beam on the synthesis of materials is studied in detail,and the similarities and differences between the two energy-carried beams are compared and analyzed.The following research findings are obtained.1.The interaction of the electron beam polarity with the magnetic medium is used to promote the growth and crystallization of iron oxide hierarchical nanostructures.For the nanodendrites:the higher the tip curvature,the faster the growth rate;the smaller the diffusion/depletion rate of the precursor,and the more asymmetric the morphology.The tip splits as the tip curvature at the edge is greater than the tip curvature at the center.For the nanospherulite,the tip splits when the tip width reaches 5.5 nm-8.5 nm.The radial growth rate of the spherulites keeps constant at the early stage.In the later stage,the growth rate is nonlinear due to the change of the front and the consumption of the precursor in the solution.At the same time,the crystal structure transforms from amorphous to ferrihydrite and then Fe₃O₄.These results provide useful information on the design and synthesis of complex structural nanomaterials.2.The space charge polarization and local electric field enhancement of electron beam is adopted to induce and regulate the reversible phase transition between lead core shell nanocrystal and amorphous phase.The reversible transformation corresponds to the energy of the electron beam.It was found that triethylene glycol（TEG）and its fragments played a key role:Under a strong beam,the fragments could be stable and combine with Pb to form an amorphous phase;under a weak beam,the fragments will combine with TEG,and Pb atoms transform to crystal core,but the shell keeps amorphous.The results are useful for the interpretation of phase transitions in catalysts with nanostructure.3.The laser-induced photochemical effect is adopted to synthesis 2D Ni/Co/Cu-based metal organic framework（MOF）structures.The precursors,TEG and metal salt,absorb laser and induce photochemical reaction,resulting in 2D MOF structure with metal cluster as active site.The as-synthesized catalysts were applied to CO₂photocatalysis.4.Selective nitrogen doping is achieved by the selective ablation reaction of the laser induces molecular fragmentation of graphene.Graphene is difficult to be doped with nitrogen,while graphene oxide is easy to be doped.The group on the surface of graphene oxide is connected with ammonia to achieve a high proportion of pyridinic nitrogen doping（51%）,which enhances the adsorption of active H.Thereby the hydrogen evolution reaction is improved remarkably.Based on the above results,the following conclusions can be drawn:the energy of the electron beam or the laser beam can be absorbed by the surface of the material,causing physical,chemical or phase structure transformation in a very short time,thereby inducing a chemical reaction for the synthesis or regulation of nanomaterials.However,the mechanism and the application scope of the two energy-carried beams are quite different.The polar electrons in electron beam induce a free radical-related reaction to synthesize nanomaterials,while the neutral photons in laser beam mainly activate precursor molecules.In addition,the polarity of the electron beam interacts with the magnetic material to promote oxidation and surface atom diffusion.The laser beam can selectively ablate materials to achieve elemental doping.