Solvent-thermal preparation of sulfur and nitrogen-doped carbon dots with PET waste as precursor and application in light-blocking film

Sulfur and nitrogen-doped carbon dots (SN-CDs) were prepared by the solvothermal method with PET waste, sulfuric acid, and phenylenediamine as precursors. The as-prepared SN-CDs were characterized by TEM, XPS, FTIR, NMR, and fluorescence spectrometer, and their applications in light-blocking membranes (LBFs) were explored. The results show that SN-CDs are spherical structures with an average particle size of 4.3 nm, and there are sulfur-containing and nitrogen-containing groups on their surface. The fluorescence emission peak for SN-CDs is dependent on the excitation wavelength in the range from 320 to 500 nm. When the excitation wavelength is 420 nm, the emission peak for SN-CDs is located at 540 nm, and the fluorescence intensity is the highest. The fluorescence quantum yield can reach 49.36%, and the average fluorescence lifetime is 6.28 ns. In terms of application, the as-prepared SN-CDs could be dispersed in polylactic acid (PLA) matrix to prepare light-blocking film (LBFs) for UV and blue light. When the amount of SN-CDs is 7%, the blocking rate of LBFs for 365 nm ultraviolet light is close to 100%, and the blocking rate for 430 nm blue light is as high as 90%. As a result, its ability to block light from mobile phone screens and the sun is better than that of commercially available glasses, and its ability to block light from mobile phone screens and the sun is excellent.