Polypropylene-Derived Luminescent Carbon Dots

Carbon dots (CDs), as an emerging material class, have been actively investigated for applications in biomedical fields, supercapacitors, and optoelectronic devices. Recently, consumer plastics have been shown to be promising starting materials for the preparation of CDs due to the high carbon percentage in their chemical composition. This also offers an upcycling pathway for waste consumer plastics that otherwise cause a significant waste management issue. Our research aims to develop a generalizable approach to upcycling plastic to carbon dots. Among all plastics, we chose to start with polypropylene as it is one of the most challenging consumer plastics to recycle or upcycle due to its excellent chemical and thermal stability. We developed an effective two-step synthesis for the preparation of carbon dots from polypropylene [1]. In the first step, bulk polypropylene is converted to nanoparticles with diameters of 310±178 nm using a dissolution/reprecipitation process. In the second step, the polypropylene nanoparticles (PP-NPs) are carbonized by hydrothermal treatment with sulfuric acid. The resulting product is polypropylene-derived carbon dots (PP-CDs) with diameters ranging from 1.8 nm to 108 nm depending on hydrothermal treatment temperature.<br/>The structure and photonic properties of the PP-CDs also vary significantly with hydrothermal treatment temperature. At a lower temperature (120 °C), the PP-CDs are large in size (~70 nm), have low photoluminescence quantum yield, and broad emission spectra, and they exhibit a small variety of surface functional groups. At higher temperatures, the product is free from unconverted polypropylene due to the combination of the high surface-to-volume ratio of the PP-NPs formed during the first step and the effective carbonization in the second step. The size of the PP-CDs formed at higher temperatures (150 °C and 180 °C) reduces significantly to ~3 nm in diameter, the quantum yield increases to 10.3 %, the emission is narrower, and the chemical composition is more heterogeneous. Powder X-ray diffraction and scanning electron microscopy also indicate the crystallinity of the post-processed CDs from 150 °C and 180 °C is high, with a graphitic-like structure. This work demonstrates an effective two-step method to fully convert polypropylene to carbon dots and shows a high degree of tunability in the size, structure, and photonic properties of the carbon dot product.<br/>[1]. Yang, Y.; Sreekumar, S.; Chimenti, R. V.; Veksler, M.; Song, K.; Zhang, S.; Rodas, D.; Christianson, V.; O’Carroll, D. M. Polypropylene-Derived Luminescent Carbon Dots. ACS Materials Lett. 2024, 6 (5), 1968–1976. https://doi.org/10.1021/acsmaterialslett.3c01419.