Investigation of the synthesis of carbon dots from citric acid and urea: evidence of the formation of organic fluorophores and study of their impact in catalysis

While carbon dots (CDs) have attracted increasing attention these last years as a new family of carbon materials with amazing optical properties and applications in ever-wider fields, many CDs described in the literature come from bottom-up syntheses using organic molecules as precursors. In particular, a very simple and rapid method largely employed consists in the microwaves treatment of an aqueous solution of citric acid and urea using microwaves. This method leads to CDs with a very high fluorescence quantum yield which are generally used for several applications such as catalysis without thorough purifications. In these works, we highlight the formation of a large quantity of molecular organic fluorophores during the synthesis protocol of these CDs. After a purification process based on chromatographic separation and Soxhlet extraction, we isolated purer CDs from the mixture of organic fluorophores. Then, we investigated the catalytic properties of these CDs and these organic fluorophores when associated with ZnO or TiO2 photocatalysts. While CDs are generally described as excellent co-catalysts for protons photoreduction, we evidenced the role of these organic fluorophores in photocatalysis performances generally assigned to CDs while the later used pure seem to deactivate the catalytic properties of semiconductors.