Impact of butanol and ammonium fluoride on synthesizing and optical properties of N-doped-carbon dots

Abstract

In the present study, nitrogen doped carbon dots (N -CDs) were synthesized by thermal reaction of citric acid and monoehanolamine (MEA) under reflux condition. The impacts of butanol, as the co-solvent, and ammonium fluoride, as the source of fluorine and nitrogen, were investigated on the optical properties of synthesized CDs. A two-level full factorial design was considered for the experiments to analyze the effect of three factors; citric acid (CA) content (0.6–1.9 mol/l), ammonium fluoride (AF) content (1.1–4.5 mol/l) and volume ratio of butanol (butanol/monoetanolamine: 0–1). The physicochemical properties of synthesized CDs were characterized through, FTIR, Raman, TEM, UV–Visible spectroscopy, Photoluminescence, XPS and HRTEM analysis. FTIR results revealed CC and C–N groups in synthesized CDs. D-band and G-band were observed in Raman spectra that proved CDs formation and AF caused more defects in CDs' structure. TEM images showed that in the presence of butanol, smaller CDs were formed with a relative wide size distribution. Absorption edge as a sign of CDs' band gap and absorption width as a sign for CDs' uniformity, were estimated from absorption spectra in UV–Visible spectroscopy. The absorption results revealed the significant effect of ammonium fluoride and citric acid on absorption edge, in such a way that it was approached to 650 nm at the maximum level of citric acid and ammonium fluoride, in this study. In addition, excitation – dependent emission were observed in synthesized CDs through PL analysis. Finally, for optimum CDs with highest absorption edge, HRTEM image indicated very small CDs (under 3 nm) and XPS results confirmed N and F doping in CDs' structure.