Leveraging self-passivation of quantum dots via nitrogen-doping for multifunctional biopolymer nanocomposites

Abstract

The plant-based and industrially compostable polymer, poly(L-lactide) (PLLA) is an indispensable substitute for petroleum-based polymers but has limited functionality owing to material and processing-related challenges. To address this, we design graphene oxide-based quantum dots (GO-QDs) with varying degrees of nitrogen-doping from thermal treatment of citric acid and urea, yielding strong and tough nanocomposites with exceptional visible-light transmittance (>70%), selective UV-visible light blocking ability, and fluorescence. For the first time, we clarify that this behaviour arises from the inclusion of carbonyl, pyrrolic, pyridinic, and graphitic-nitrogen in QDs with an increasing degree of nitrogen-doping, each offering distinct light absorption tendencies. The inclusion of just 0.25 wt% of synthesized QDs achieves a 43.4% improvement in toughness with a concomitant increase in stiffness and strength of 13.7% and 13.3%, respectively. This behaviour is attributed to increased craze density confirmed by fractography analysis. Furthermore, GO-QD inclusion can improve the processability of PLLA, increasing its crystallization temperature by over 10 °C, while reducing melt viscosity by up to an order of magnitude owing to size and composition effects, which are valuable in downstream manufacturing processes. This unique combination of properties cannot be achieved with existing micro/nanoparticle UV-absorbers (e.g., zinc oxide, titanium dioxide), reinforcing additives (e.g., cellulose derivatives, polymer nanofibrils), and processing aids (e.g., nucleating agents, plasticizers). This study not only sets a new benchmark for the optical, mechanical and processing-behaviour of PLLA, but also demonstrates the transformative potential of nitrogen-doping in expanding the functional capabilities of green polymer nanocomposites.