Aerosol synthesis of flame formed carbon dots: Insights of size, morphology and optical properties

Abstract

Particle size of carbon dots (CDs) below 10 nm exhibits distinct features compared to larger particles. The size dependent features of CDs significantly contribute to their overall morphology, composition, and optical properties. Previously, theoretical models have predicted the synthesis of flame formed carbon dots which potentially demonstrate quantum confinement effect; however experimental studies are rare in this context. Especially, aerosol synthesis of flame formed CDs have not been explored yet. This study demonstrates synthesis of CDs through flame spray pyrolysis (FSP); using either an FSP open system (FSP-OS) or FSP closed system (FSP-CS). Their respective properties including size, morphology, and optical features were investigated. The entrainment of ambient air in the open system, and the equivalence ratio used in the closed system, were found to have major impact on the particle number concentration of the CDs. Moreover, these factors also controlled the overall agglomeration process and primary particle size. Carbon dots produced through closed system (CDs-CS) exhibited a more graphitic nature compared to those from open system (CDS-OS) owing to the presence of thermally more stable and structured forms of elemental carbon (EC). This is also evidenced by their TEM results and Raman intensity ratios. Furthermore, CDs-CS also demonstrated enhanced visible light absorption. The band gap of the EC fraction was 1.56 eV which implies that these are incipient CDs and extend the light absorption from visible to near infrared region. Whereas the presence of different polyaromatic hydrocarbons in the organic carbon fraction leads to variation of band gap in the range 1.75–4.65 eV and 1.84–4.58 eV for CDs-CS and CDs-OS, respectively.