Carbon dots (CDs) are an emerging branch of carbon-based nanomaterials receiving attention due to their attractive structural and optical properties. CDs are mostly spherical nanoparticles composed of a carbonaceous core (amorphous or crystalline) with various functional groups at the edge, making them a colloidal system that exhibits good dispersibility in a wide range of solvents. Their outstanding optical properties, such as strong absorption and tunable emission wavelength, together with low cost and low toxicity, making CDs great candidates for various applications from biomedical to energy conversion and storage. However, the origins and fundamental mechanisms of their photoluminescence (PL) remain unclear as the structure of CDs is heterogeneous and is largely determined by the preparation routes which vary widely. Additionally, most tunable PL studies focus on small organic molecule-derived CDs, such as citric acid and polycyclic aromatic hydrocarbon compounds. The potential of another carbon-rich precursor, i.e., plastic polymers, has been largely underestimated despite an abundant source. In addition, the cleaning and recycling of plastic has been a significant concern in our community. Therefore, developing an efficient methodology to convert plastic polymers into luminescent CDs will greatly improve the carbon cycle and reduce the plastic waste footprint. Furthermore, there are few studies on the structural and property changes during the solidification of CDs, including the effects of applied heating or pressure during the solvent removal process, leaving a large gap between the academic research and realistic industrial applications. In this dissertation, first, we explore the effect of carbon core crystallinity and size on the PL of CDs prepared from citric acid and urea with a microwave-assisted synthesis. We control the crystallinity of the carbon core by addition of phosphoric acid. By controlling the microwave power, further size control of CDs is achieved. We establish the origins of the photophysical properties of as-prepared CDs by analysis of structure and multiple PL properties. Carbonaceous core crystallinity is correlated with an extended PL lifetime and elevated PL quantum yield (QY) (up to 14.6%). Size decrease and higher defect density at the surface were found to lower the QY, attributed to increased non-radiative decay from bond vibrations. Our findings establish a mechanistic framework for the dual influence of crystalline and size on the photophysical properties of CDs and provide design guidelines for optimizing luminescence in carbon-based nanomaterialsThen, we demonstrate an effective route to prepare luminescent CDs from polypropylene (PP). Our invention of a two-step process, combining non-solvent induced phase separation and hydrothermal reaction under strong acidic and oxidative environment, successfully form luminescent CDs from both commercial PP pellets and real-life PP waste. Further exploration of the hydrothermal temperatures disclosed that it determines the product size, chemical composition and photophysical behaviors. At lower temperature (120 °C), the PP-CDs are large in diameter (~60 nm), have low quantum yield, broad emission spectra and they exhibit higher carbon content, especially sp3 C with a small variety of surface functional groups. In contrast, at higher temperature (150 °C and 180 °C) the size of the PP-CDs reduces significantly to ~3-5 nm, the quantum yield increases to 10.3 %, the emission is narrower, and the chemical composition is more heterogeneous. This work demonstrates an effective two-step method to fully convert PP to luminescent CDs and exhibits a high degree of tunability in the structural and photonic properties of the CDs product.Finally, we investigate the self-assembly behavior of CDs from a sub-nanometer scale to micrometer scale graphitic structures. We show that drying conditions and aqueous aging direct the transformation of the individual CDs to lamellar sheets with different interlayer distance and surface expansion. Freeze-drying (FD) preserves a hydrated, weakly hydrogen-bonded lamellar framework, while rotary evaporation drying (ED) concentrates the dispersion and drives crystallization of low-molecular-weight residues without generating substantial π-stacking. A combination of ED and FD were found to be sufficient to induce the graphitic domains. An additional aqueous aging for 1-2 days before drying can significantly facilitate closer stacking between layers by molecular crystal dissolution and hydrolysis of organic fragments. Specially in ED, and EDFD routes, large sheet crystals with strong PL under 470 nm excitation were observed, aligned with the increased π-π stacking and heteroatom doping in the carbonaceous core. This insight provides a framework for rational design of solid-state carbon materials by controlling stacking order, functional-group distribution, and structural uniformity derived from polymer precursors.
Yongqi Yang (Thu,) studied this question.