Preparation Of Graphene Quantum Dots And Study Of Their Fluorescence Properties

Graphene,a single layer of carbon atoms formed in honeycomb lattice,is a zero-bandgap material.Graphene sheets smaller than 100nm show quantum confinement behaviour which generates band gaps in their electronic band structures.Converting 2D graphene to OD graphene quantum dots(GQDs)is effective approach to yield an emerging photoluminescence due to their remarkable quantum confinement and edge effects.In this thesis,We report a pyrolytic method to synthesize GQDs by pyrolyzing citric acid,to explore the influence of synthesis conditions,solvent,Ag nanoparticles on the optical properties of GQDs.This paper mainly carried out the following work:1.We prepare photoluminescent GQDs in different synthesis conditions by using the citric acid as the carbon source through pyrolysis process.The result show that the fluorescence of as-prepared GQDs is dependent of excitation wavelength in the range of 180-200?,5-20min.As the increase of pyrolytic temperature or durations,the fluorescence of GQDs is gradually broadened and weakened,the strongest fluorescence peak of GQDs exhibits a red-shift.Over high pyrolytic temperature or overlong durations can increase the degree of carbonization of citric acid and the size of GQDs,decrease the intensity of fluorescence.The fluorescence of GQDs is also pH-dependent.Under neutral conditions,GQDs emit strong fluorescence,while the fluorescence will be weakened in acidic or alkaline solution.2.The GQDs were prepared by pyrolyyzing citric acid and dissolved with different organic solvents.The results show that the average sizes of GQDS are 4.9nm and these GQDS consist of 1-5 graphene layers.The photoluminescence spectra depict that the GQDs exhibited solvent-dependent behaviors.The fluorescence of GQDs in a nonpolar solvent pentane is independent of excitation wavelength,the strong emission appeared in the near-UV region.The position of the peak fluorescence of GQDs in polar solvent ethanol,acetone is dependent on excitation wavelength,emission wavelength is in a visible light area.When GQDs is in a polar solvent and excited by a light source,a slow solvent relaxation process occurs on the same time scale as the fluorescence emission,inducing a "giant red-edge effect".The giant red-edge effect disappears in a nonpolar solvent,leading to a fluorescence peak independent of the excitation wavelength.As a result,the fluorescence of GQDs in pentane exhibit a relatively narrow bandwidth and excitation-independent behavior.3.GQDs aqueous solution with the concentration of lmg/mL was prepared by thermally decomposed citric acid.Different volume of AgNO3 with a concentration of 0.1mol/L were divided into GQDs aqueous solution.In the presence of UV irradiation,AgNO3 in GQDs would decompose to form Ag.GQDs-Ag display content-dependent features.The fluorescence of GQDs-Ag is gradually weakened with Ag content increased,but the fluorescence peak position is independent on the Ag content and excitation wavelength.