Selective Optimized-Plasma Synthesis of Bio-Resource-Derived Nanographene and Nanodiamonds

Carbon quantum materials such as graphene quantum dots and nanodiamonds have promising applications in bio-imaging, energy conversion, storage, optoelectronics, and nanocatalysis because of their unique properties, including tunable photoluminescence, quantum confinement, and biocompatibility. However, their precise nanoscale engineering is challenging due to inefficient synthesis methods and a limited understanding of their growth mechanisms. To overcome these obstacles, we have developed a DC atmospheric-pressure microplasma-assisted electrochemical method, enabling catalyst-free reactions through higher electron density at the plasma-liquid interface and allowing for precise tuning of graphene quantum dots' structures and optical properties. This method demonstrates that sustainable bio-resources can serve as carbon sources without requiring high temperatures or vacuum conditions. We have employed non-intrusive in-situ optical emission and absorbance spectroscopy as a diagnostic tool to understand the underlying growth mechanisms of these materials.<br/>We have unveiled a significant correlation between the production, emission properties, and chemical phases of graphene quantum dots (GQDs) and the concentration of OH radicals in the plasma, which the discharge parameters can control. Notably, we have observed that higher OH radical concentrations enhance the dissociation of the precursor molecule, leading to an increased production of GQDs. Our studies using TEM and Raman techniques have yielded valuable insights into the composition of graphene quantum dots produced from fructose. We found a combination of graphene and diamond structures and observed that the sp³-to-sp² ratio can be adjusted through plasma energy. By increasing the precursor concentration and discharge current and optimizing discharge parameters, we can enhance emission intensity, leading to faster growth rates and improved optical properties. It was also observed that plasma conditions can influence the carbon phases, with longer plasma treatment times causing a shift from sp² to sp³. Furthermore, our research indicates that higher plasma currents enhance plasma emission, resulting in increased production of graphene quantum dots by generating more OH radicals. This research contributes to the advancement of plasma-assisted synthesis methods and provides a basis for the scalable and controlled manufacturing of high-quality CQMs. These insights are crucial for designing customized nanomaterials for integration into a wide range of advanced technological applications.