Carbon Dioxide Capture by Carbon Dots as a Promising Approach to Reduce Global Warming

Climate change is a major crisis that affects human lives with a detrimental impact on decades of progress in global health. As a major contributor to the climate change, global warming that is mainly attributed to CO2 emission demands an effective mitigation strategy. Under such a circumstance, nanomaterials with high surface-area-to-volume ratios are promising in CO2 capture. As a relatively new family member of carbon-based nanomaterials, carbon dots (CDs) that possess small particle sizes (1-10 nm) but high surface-area-to-volume ratios and abundant surface functional groups such as amine and hydroxyl groups are particularly favored for this application. In this study, four types of CDs, namely carbon nitride dots (CNDs), gel-like CDs (G-CDs), pentaethylenehexamine (PEHA)-derived CDs (P-CDs), and yellow-emission CDs (Y-CDs), were synthesized by following their respective well-established protocols. They were characterized by UV/vis absorption, fluorescence emission, Fourier-transform infrared spectroscopies, and atomic force microscopy. It is noteworthy that when a controlled synthesis was conducted for CNDs, a unique fluorescence behavior was observed. Then all the CDs were tested for CO2 adsorption capacity with the help of thermogravimetric analysis, and the CO2 adsorption capacity of the CDs in natural state was arranged in the order of G-CDs (66.7 mg/g) > P-CDs (28.3 mg/g) > Y-CDs (10.0 mg/g) > CNDs (6.7 mg/g). In order to accurately analyze the CO2 adsorption capacity of G-CDs and determine whether it is amine or hydroxyl group, that plays a dominant role in the CO2 capture, TGA was performed on the demoisturized, and modified G-CDs, respectively, which showed that G-CDs possess a high CO2 uptake capacity of 56.7 mg/g that is determined by both functional groups. Subsequently, as a demonstrated photocatalyst material, G-CDs were applied to efficiently convert CO2 into organic matter. In conclusion, this study reveals the structure-property relationship of CDs in CO2 capture and conversion. Significantly, it demonstrates a great potential of CDs in the global warming reduction as well as affiliated applications in agriculture and cosmetics.