Properties of Carbon Dots Synthesized Solvothermally from Citric Acid and Urea

We propose a simple one-stage method to prepare carbon dots using citric acid as a carbon source and urea as a nitrogen source. It is shown that luminescence spectra of carbon dots depend on the excitation wavelength. When excited by light with a wavelength from 300 nm to 550 nm, these spectra are shifted by a value from 440 nm to 626 nm. The absorption band is formed by the n-p* transition in the C-O bond and by the π-π* transition of aromatic sp2 domains (C-C) in the region of 200–270 nm and by the π-π* transition of π systems containing C-N/C-N bonds in the region of 550–570 nm. The infrared absorption spectra and X-ray photoelectron spectra demonstrate the presence of C-C, C-N-C bonds, epoxide (C-O-C), and hydroxy (-OH) functional groups which directly affect the red shift of luminescence spectra. Raman spectra indicate a presence of both sp2 and sp3 carbon structures. Scanning and transmission electron microscopy data indicate that the sizes of carbon dots vary from 2 nm to 100 nm.