Effect of Concentration of Citric Acid on Size and Optical Properties of Fluorescence Graphene Quantum Dots Prepared by Tuning Carbonization Degree

Abstract

In this work, the effect of concentration of citric acid precursor on size and optical properties of graphene quantum dots has been investigated. Citric acid precursor (CA) was obtained at different concentrations of 0.1, 0.2, 0.5, and 1.0 M by dissolving citric acid monohydrate powder in ethanol. The CA was then carbonized to form graphene quantum dots (GQDs) at a temperature of 250 ฐC. Based on observation by naked eye, the color of CA changed from an initial colorless solution to melted yellow and it exhibited strong blue fluorescence under excitation wavelength of 365 nm. These results implied formation of GQDs. Characterizations of GQDs were performed by using dynamic light scattering, FT-IR spectroscopy, UV-vis spectroscopy, and fluorescence spectroscopy. The typical morphology of GQDs was carried out by utilizing transmission electron microscopy. It was found that average size of GQDs decreased on elevating the CA concentration. The absorbance spectra exhibited strong excitonic absorption bands in ultraviolet region and their absorption edge decreased from 327 to 314 nm on increasing the CA concentration. The fluorescence spectra exhibited bright fluorescence bands in visible region and their peak fluorescence decreased from 459 nm to 443 nm with increasing CA concentration. Optical properties of GQDs confirmed their non-zero band gap and tunable fluorescence property via synthesis condition. Accordingly, size-dependent optical properties of fluorescence GQDs could be explained in term of quantum size effect. These results potentially have novel applications in bioimaging, biosensing, and light emitting diodes.