Carbon quantum dots derived from waste acorn cups and its application as an ultraviolet absorbent for polyvinyl alcohol film

Highlights: • Biomass CQDs were synthesized from acorn cups via one-step hydrothermal method. • Nitrogen-doped CQDs is functionalized by the ethylenediamine. • The prepared CQDs can effectively absorb ultraviolet light and transform it. • The fabricated PVA films with chitosan/CQDs have excellent UV-blocking properties. Extracting effective ultraviolet (UV) absorbent from biomass-based material is urgent for the sustainable development. Here, non-doped CQDs (1-CQDs) and nitrogen-doped CQDs (2-CQDs) were synthesized from acorn cups (ACs) with a green method. The active surface groups of 1-CQDs (4.16 nm) include Ph-OH, OH, and COOH, etc., while for 2-CQDs (5.23 nm), nitrogen doping provides a large number of nitrogen atoms, so that the active functional groups include CNH₂, CNC, CNH and CONH, etc. The carbon network of 1-CQDs is dominant for blue emission, the subordinate blue-green emission is derived from ellagitannin. For 2-CQDs, green emission comes from the flavonoid produced during hydrothermal carbonization and nitrogen doping processes. We combined chitosan (CS) into CQDs to synergistically act as UV absorbents for functionalizing PVA films. The UV-A blocking capacity and transparency of the PVA/CS/1-CQDs film are 83.58 % and 79.34 %, respectively. The PVA/CS/2-CQDs film can block UV-A up to 94.53 %. This study provides a new way for the clean and high-valued-added utilization of waste ACs, discusses the process and mechanism of extracting CQDs from ACs, and applies it to UV light conversion system of PVA film. It presents a reference for the development of a new generation of UV absorbents.