Irradiation-Assisted Hydrothermal Conversion of Water Hyacinth Leaves to Fluorescence Carbon Dots with High Quantum Yield

Electron beam irradiation (EBI) (100−400 kGy) was utilised to pretreat water hyacinth (WH). Scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), fourier transform infrared (FTIR), UV-vis absorption and fluorescence spectroscopy were used to examine the effects of EBI and hydrothermal temperatures on the physical, chemical and optical characteristics of carbon dots (CDs). The combination of 400 kGy-pretreatment and 250°C-hydrothermal treatment as the optimal condition for CDs synthesis provided the highest quantum yield (QY) of 14.5% and the product yield of 7.5%. Using dynamic light scattering (DLS), the CDs showed an average size of around 180 nm, and the zeta potential analysis revealed the overall negative charge on the CDs’ surface. In comparison, the CDs synthesised via hydrothermal conversion at 250°C without EBI showed a QY of only 4.0% and a product yield of 3.8%, significantly lower than the condition with 400 kGy-pretreatment. The improved optical characteristics of the CDs could result from the high nitrogen contents of WH biomass, oxygen-rich surfaces from EBI pretreatment at 400 kGy and the aromatic skeleton of the CDs.