Graphene quantum dots (GQDs) containing N atoms were successfully synthesized using a facile, inexpensive and environmentally friendly hydrothermal reaction of urea and citric acid and the effect of the GQDs’ C–N configurations on their photoluminescence (PL) properties were investigated. High-resolution transmission electron microscopy (HR-TEM) images confirmed that the dots were spherical, with an average diameter of 2.17 nm. X-ray photoelectron spectroscopy (XPS) analysis indicated that the C–N configurations of the GQDs substantially affected their PL intensity. Increased PL intensity was obtained in areas with greater percentages of pyridinic-N and lower percentages of pyrrolic-N. This enhanced PL was attributed to delocalized π electrons from pyridinic-N contributing to the C system of the GQDs. On the basis of energy electron loss spectroscopy (EELS) and UV-Vis spectroscopy analyses, we propose a PL mechanism for hydrothermally synthesized GQDs.

Carbon-based Quantum dots (C-QDs) are carbon-based materials that experience the quantum confinement effect, which results in superior optoelectronic properties. In recent years, C-QDs have attracted attention significantly and have shown great application potential as a high-performance supercapacitor device. C-QDs (either as a bare electrode or composite) give a new way to boost supercapacitor performances in higher specific capacitance, high energy density, and good durability. This review comprehensively summarizes the up-to-date progress in C-QD applications either in a bare condition or as a composite with other materials for supercapacitors. The current state of the three distinct C-QD families used for supercapacitors including carbon quantum dots, carbon dots, and graphene quantum dots is highlighted. Two main properties of C-QDs (structural and electrical properties) are presented and analyzed, with a focus on the contribution to supercapacitor performances. Finally, we discuss and outline the remaining major challenges and future perspectives for this growing field with the hope of stimulating further research progress.

Carbon dots (CDs) have attracted great attention because of their unique luminescence properties, chemical inertness, thermal stability, high water solubility, low toxicity, and ease of functionalization. Here, the kinetics of nitrogen-doped CD (N-CD) formation by hydrothermal synthesis were evaluated in an attempt to realize the rapid and efficient production of N-CDs. A series of N-CDs was synthesized using various heating rates, reaction times, reaction temperatures, and precursor concentrations. Characterization of the series of N-CDs indicated that N-CD formation is a first-order reaction with a reaction rate constant of 0.634 min⁻¹. In addition, systematic investigation revealed that synthesis temperature is a more important factor to obtain highly fluorescent N-CDs than reaction time. Citric acid amides are formed by the reaction between citric acid and urea at 130 °C and N-CDs consisting of two or three citric acid amide molecules are formed through dehydration, deammoniation and dehydrogenation of citric acid at 150 °C. By adjusting the operating conditions, N-CDs with a highest quantum yield of 39.7% could be produced at a production rate of 50 g h⁻¹ with a reaction time of 16 min. The N-CDs were then embedded in polyvinyl alcohol (PVA) nanofibers. The luminescence intensity of the N-CD–PVA composite nanofibers was more than twice that of the N-CDs in solution.

By rationally designing and modifying their heterostructures, carbon dots have the potential to be used as solar absorbers for solar evaporation. The roles of carbon dots to improve solar-to-vapor efficiency are thoroughly discussed.