Environmental-friendly carbon quantum dots (CQDs) nanofluid was synthesized using microwave heating of polyethyleneglycol 200 (PEG 200). PEG acts as a carbon source, base fluid and dispersant simultaneously. The synthesized CQDs were characterized using transmission electron microscopy (TEM), dynamic light scattering (DLS) and fluorescence spectroscopy. The synthesized CQDs with average diameter of 6.5 nm were solvated with PEG molecules, and consequently, the nanofluid with an excellent long-term stability was obtained. The synthesized CQDs nanofluid was used for photo-thermal conversion. The infrared thermographs showed that the surface temperature of nanofluid reaches to >60 ◦C after irradiation using LED lamp with 0.8 Sun irradiance for 100 min. The synthesized nanofluid was used at several consecutive heating/cooling cycles without losing its efficiency in photo-thermal conversion. Total absorbed energy, stored energy ratio, photo-thermal conversion efficiency and rate constant of heat dissipation were calculated and discussed in details. Because of stability at high temperatures (up to 200 ◦C), preservation of photo-thermal efficiency at consecutive heating/cooling cycles and longtime durability, the synthesized CQDs nanofluid can be considered as a super-stable nanofluid. Additionally, the performance of the CQDs nanofluid in solar energy harvesting and its application for water heating in a continuous flow system was examined. All results confirm that the synthesized CQDs nanofluid has many advantages including low cost and simplicity of preparation procedure, no need to use dispersant, long-life stability, very high thermal-stability and high photo-thermal conversion efficiency. Therefore, CQDs nanofluid has high potential to be utilized for practical solar energy harvesting.
