A review on photocatalytic CO2 reduction of g-C3N4 and g-C3N4-based photocatalysts modified by CQDs

The environmental problems caused by industrial development are becoming prominent, the most urgent environmental concern is that the CO2 concentration in the atmosphere has exceeded the safe level and even continuously rising. Photocatalysis technology can directly convert CO2 into high-value-added chemicals such as CO, CH4, and CH3OH with solar energy, which is beneficial for alleviating environmental problems and reducing resource waste. Graphitic carbon nitride (g-C3N4), a non-metallic catalyst, has been widely used in photocatalytic CO2 reduction due to its narrow band gap, suitable band location, high abundance, low cost, and simple synthesis. However, the application of g-C3N4 as a catalyst is limited due to the low utilization rate of visible light and the high recombination rate of electron holes in the reaction process. Carbon quantum dots (CQDs) became a promising photocatalyst based on adsorbing more CO2, excellent up-conversion luminescence, and transferring electrons. The recent progress of CQDs modification on g-C3N4 and g-C3N4-based catalysts in photocatalytic CO2 reduction was reviewed in this review, the mechanism of photocatalytic CO2 reduction is described, then the synthesis method and structure of CQDs are introduced, the effect of CQDs modification on g-C3N4 and g-C3N4based catalysts in light utilization efficiency, CO2 molecular adsorption capacity, and CO2 reduction capacity of the photocatalytic reaction was discussed in detail in finally. The challenges and future perspectives of CQDsmodified g-C3N4 and g-C3N4-based photocatalysts in photocatalytic applications were given. We hope that this review will provide a valuable overview and insight for the promotion of applications of CQDs-modified g-C3N4based photocatalysts.