Rapid Self-assembly Of Three-dimensional Photonic Crystal Using Fe₃O₄@C Magnetic Composite Nanoparticles And Its Application

The preparation of photonic crystals by self-assembly of magnetic composite nanoparticles?MCNPs?is a classical technique.Owing to small sizes,special magnetic properties,and easiness of modification,the MCNPs are suitable as the building blocks to construct photonic crystal structures.Photonic crystals are composed of periodic dielectric microstructure or nanostructure with photonic band gaps,and are widely applied in many fields because of their unique optical diffraction behavior and structural color features.Therefore,it is indispensable to find a rapid method to self-assemble the MCNPs into photonic crystals and extend them to practical applications.The thesis aimed to explore a rapid self-assembly method to construct photonic crystal structures based on the MCNPs as the building blocks,to prepare functional photonic crystal materials,and to develop optical films and sensors in practical applications.The main contents of the thesis were as follows:1.With the solvothermal synthesis,the monodisperse core-shell carbon-coated ferroferric oxide magnetic composite nanoparticles?Fe₃O₄@C MCNPs?were prepared,and the effects of reaction temperature and hydrogen peroxide?H₂O₂?concentration on the size and morphology of nanoparticles were investigated.It was found that the concentration of H₂O₂ was mainly related to the shell thickness and surface charge of the particles.As the H₂O₂ concentration increased,both the shell thickness and surface charge decreased.Whereas,the reaction temperature was mainly related to the particle size,the particle size increased with the increase of reaction temperature.By controlling the reaction temperature between180-200?,the monodisperse Fe₃O₄@C MCNPs with an average particle size of 201-261 nm were prepared.During the solvothermal synthesis process,the Fe₃O₄@C MCNPs with weak ferromagnetism were arranged to form the permanent nanochain structure under the equilibrum between the magnetic attraction and electrostatic repulsion forces.So,when the Fe₃O₄@C MCNPs were dispersed in a solvent,they could form photonic crystal structure.By changing the external magnetic field strength or the angle of incident light,the photonic crystal dispersed solution could diffract bright colors covering the entire visible light spectrum.2.With the evaporation-induced self-assembly method,the solid-phase three-dimensional photonic crystals were prepared based on rapid self-assembly of Fe₃O₄@C MCNPs,and the influences of deposition conditions on the photonic crystal structure were investigated.It was found that deposition conditions such as deposition temperature,Fe₃O₄@C dispersed solution concentration,and the type of the solvent had significant influences on the photonic crystal structure.Through single-factor experiments,the optimum deposition conditions were determined as follows:the deposition temperature of 90?,the solution concentration at 1.88 mg/mL and deionized water as the solvent.The photonic crystals prepared under optimal deposition conditions had regular structure,bright diffraction colors,and distinct metallic luster,and it took only 2.25 h by self-assembly.During the self-assembly process,the one-dimensional nanochains in Fe₃O₄@C dispersed solution stacked quickly to form the regular solid-phase three-dimensional photonic crystal structure under the combined action of both the capillary force and the magnetic interaction force between the nanoparticles.The diffraction color of the solid-phase three-dimensional photonic crystal structure was related to the spacing between the nanochains.The Fe₃O₄@C solid-phase three-dimensional photonic crystals constructed with Fe₃O₄@C MCNPs with particle sizes of 201nm,230 nm,and 261 nm,respectively,could diffract the color of green,yellow,and red correspondingly.3.Chitosan?CS?,polyvinyl alcohol?PVA?and polymethyl methacrylate?PMMA?,respectively,were chosen as film formers,to prepare photonic crystal films by the evaporation-induced self-assembly of Fe₃O₄@C MCNPs.The optimum film-formed conditions were investigated.On the basis of the 0.015 g Fe₃O₄@C MCNPs,Fe₃O₄@C/CS photonic crystal film was prepared with the 0.4wt‰CS,and Fe₃O₄@C/PVA photonic crystal film could be formed with the volume ratio of 4:4,4.5:3.5 or 5:3 of ethanol?or acetone?to3wt‰PVA aqueous solution system.Fe₃O₄@C/PMMA photonic crystal film was in-situ polymized with monomer MMA of 500?L/4.9 cm².All these photonic crystal films had bright diffraction colors and distinct metallic luster.4.With thermal-initiated polymerization method,the polyacrylamide-based photonic crystal gels were constructed using Fe₃O₄@C MCNPs as building blocks,acrylamide as the monomer,N,N'-methylenebisacrylamide as cross-linking agent,and ammonium persulfate as thermal initiator.The responsiveness of photonic crystal gels to the external stimuli such as pressure,humidity and formaldehyde were investigated.The gels could obviously respond to the above-mentioned external stimuli in the manner of their own color change.The response process was visible to the naked eye,and had the characteristics of sensitivity and reversibility.These characteristics enabled the photonic crystal gels to be used as pressure sensors,humidity sensors and formaldehyde sensors based on their own color change,which makes the visual detection a reality.