Impact of sodium doping on the structural properties of CQDs

Nitrogen- and sodium-doped carbon quantum dots (CQDs) were synthesized from citric acid monohydrate and urea, using varying concentrations of Ethylenediaminetetraacetic acid disodium salt dehydrate (EDTA) in a bottom-up approach with microwave irradiation. First, a comparative study of synthesis duration and EDTA concentration was conducted. Upon optimization, our results showed that E1-CQD@300sec exhibited the highest absorbance among all EDTA-CQD samples, with absorbance values following this trend: EDTA-CQD@300sec > EDTA-CQD@225sec > EDTA-CQD@165sec for most EDTA concentrations. Additionally, photoluminescence (PL) was found to follow the order: E2-CQD@165sec > E1-CQD@225sec ≈ E1-CQD@300sec. E1-CQD@300sec had a spherical morphology with an approximate size of 13 nm, as observed in high-resolution transmission electron microscopy (HR-TEM). This E1-CQD@300sec sample has a bandgap of 1.95 eV, with a highest occupied molecular orbital (HOMO) of -5.15 eV and a lowest unoccupied molecular orbital (LUMO) of -3.2 eV. X-ray diffraction (XRD) revealed a broad peak at ~ 26.45°, indicating an amorphous structure, which was confirmed by Raman spectroscopy, showing an I_D/I_G ratio of 1.11. FT-IR analysis indicated the presence of nitrogen-based functional groups, such as C-N and N-H, on the surface. With increase in EDTA concentration, the bandgap of EDTA-CQD@300sec increased, reaching a maximum of 2.29 eV for E5-CQD@300sec, with a HOMO of -5.55 eV and a LUMO of -3.26 eV, and other functional groups, such as C ≡ C and C ≡ N, were also detected in the FT-IR spectra. In addition, agglomeration was further increased in the presence of EDTA, possibly due to uncontrolled nucleation, EDTA-CQD complex formation, high EDTA concentration, and insufficient surface coverage by EDTA, as observed through dynamic light scattering (DLS). The thermal stability of ETDA-CQD@300sec also found to decreased from ~  208°C to 40 °C. Further modifications and optimization is necessary for potential use of synthesized CQDs in the opto-electronic applications.