Concentration-dependent multi-color humic acid-based carbon dots for luminescent polymer composite films

Color-tunable luminescent polymer composite materials are desired for applications in light-emitting diodes. Herein, we directly use commercial humic acid as precursor to fabricate carbon dots by one-step hydrothermal method in formic acid and hydrogen peroxide solution. It is found that the size distribution of synthesized carbon dots ranges from 2 to 5 nm, and the carbon dots display concentration-dependent multi-color luminescence in formic acid, N,N-dimethylformamide and deionized water under 365-nm UV light, separately. The quantum yield of the as-made carbon dots with broad color tunability (445–587 nm) in formic acid is about 20% at 0.3125 mg/ml. Furthermore, by adjusting the concentration of carbon dots in polyvinyl alcohol, the composite films emit blue, cyan and light yellow fluorescence and the Commission Internationale de L’Eclairage color coordinates vary from (0.19, 0.23), through (0.25, 0.35) to (0.29, 0.40) at the content of CDs 3, 5 and 7 wt.% under the excitation of 365 nm, respectively. Meanwhile, the transmittance of the composite film at 5 wt.% is maintained at 81.8 to 87.2% of the pure polyvinyl alcohol film at 550 nm wavelength. This work provides an innovative, simple and efficient way to prepare concentration-dependent polychromatic carbon dots composite films, and the obtained multi-color light-emitting composites have a wide range of photoluminescence spectra, covering most of the visible light range, which opens up the possibility of applying these materials in the fields of optoelectronic materials. Graphical Abstract An innovative, simple and efficient way to fabricate concentration-dependent multi-color CDs/PVA composite films, which promote the application of CDs in the fields of optoelectronic materials.