Research On Structural Modulation Of Carbon-based Nanomaterials And Their Optical And Nanofiltration Properties

Carbon-based nanomaterials,which form a six-membered ring lattice structure with carbon sp2hybridization,have excellent optical,electrical,and mechanical properties and hold important promise for applications in optical devices,catalysis,micro and nano processing and energy.For example,carbon dots（CDs）with zero-dimensional nanometer size have stability fluorescence,electron transfer,biocompatibility and low toxicity,and are widely used in biomedicine,optoelectronic devices and photocatalysis.Another class of carbon-based nanomaterials are two-dimensional graphene oxide sheet layers with single-atom thickness.It can be self-assembled into graphene oxide nanofiltration membranes,demonstrating ultra-fast water permeances and high ion retention rates,among others.The wide range of potential applications of carbon-based nanomaterials in different fields cannot be separated from the precise regulation of the structure,systematic characterization of the physicochemical properties,in-depth analysis of the mechanism and application research of carbon-based nanomaterials.For CDs with excellent luminescence properties,the photoluminescence properties of emitting blue and green light are currently dominant.Synthesizing CDs with emission center to the whole visible wavelength range remains the key and challenge to realize CDs widely applicable to luminescent nanodevices.Based on the existing defects of CDs with monochromatic luminescence and short wavelengths（e.g.,blue or green light）,this thesis prepared multi-color fluorescent CDs with fluorescence covering to the whole visible wavelength band（450 nm-640 nm）by a convenient one-step hydrothermal reaction method with optimized different solvent ratios.Transmission electron microscopy（TEM）and X-ray photoelectron spectroscopy（XPS）characterizations indicate that the gradual increase of the particle size of CDs and the change of the surface functional group content make the fluorescence wavelength of CDs gradually red-shift,which indicates that the quantum size effect and the surface defect state effect together cause the shift of the emission wavelength of CDs.In addition,the variation of the emission wavelength of CDs was studied by varying other factors（e.g.,the duration of heating,the temperature of heating,and different solvents）.The result is that the emission wavelength of CDs is red-shifted with increasing heating time and temperature.These six solvents were found to be red-shifted by affecting the n-π*fluorescence emission centers of CDs when they were dispersed in weakly polar solvents from strong to weak polarity.while they are not shifted in relatively strong polar solvents.The study provides a potential reference for the fluorescence emission mechanism of CDs.Nanofiltration membranes formed by self-assembly of two-dimensional reduced graphene oxide.they are expected to be the next generation of ultra-high permeability nanofiltration membranes due to their advantages of high efficiency resulting from low energy consumption and large specific surface area compared to traditional separation techniques（e.g.distillation and evaporation）,which have attracted the research interests of scholars.Graphene membranes can control interlayer spacing with ultra-high precision（?level）,enabling precise sieving of mixed ions with very small size differences（as small as 1?or a few?）,such as K⁺,Na⁺,Ca²⁺and Mg²⁺.In this thesis work,the interlayer spacing of Amino-Hydrothermal Reduced Graphene Oxide（AH-r GO）thin films was tuned by cation-πto achieve the sieving of metal ions of different valence states of the same element(Fe³⁺/Fe²⁺,Cu²⁺/Cu⁺and Cr³⁺/Cr²⁺)with smaller size differences.Density Functional Theory calculations and experimental results show that The cation-πinteraction between the higher valence hydrate ions of the same metal element and the AH-r GO lamellae is much stronger than in the lower valence state,resulting in the higher valence hydrate ions of the same metal element controlling the layer spacing of the AH-r GO membranes lamellae more than the lower valence hydrate ions and the higher valence hydrate ions deforming in the AH-r GO membranes lamellae.The higher valence metal ions with larger hydration radii are repelled and the lower valence metal ions with relatively smaller hydration radii are allowed to penetrate during sieving of mixed salt solutions of different valences of the same element.This work achieves the limit of minimum resolution of ion sieving,which has potential applications in the fields of quantitative analysis of ions in different valence states,preparation of functional materials,and water treatment.