Nanofluid is an ideal working fluid for direct absorption solar collectors (DASCs) and a promising technology for solar energy harvesting. An advanced composite of reduced graphene oxide (rGO) nanosheets and nitrogen-doped carbon quantum dots (N-CQD) was prepared by using a hydrothermal technique. The X-ray photoelectron spectroscopy (XPS) analysis confirmed the presence of N-CQD on the surface of rGO, resulting in the formation of a rGO/N-CQD composite. Additionally, transmission electron microscopy (TEM) analysis verified the uniform distribution of N-CQD on rGO nanosheets. Physicochemical property investigations reveal that the rGO/N-CQD60/40 nanofluid exhibits the highest thermal conductivity (TC) with proper stability. An artificial neural network (ANN) was developed to estimate the TC of the rGO/N-CQD60/40 nanofluid using temperature and concentration as inputs. The model accurately predicted the TC, achieving a coefficient of determination (R2) of 0.9974. The composite of rGO/N-CQD60/40 greatly enhances optical absorption compared to GO and N-CQD. The rGO/N-CQD60/40 nanofluid, which achieves 82.64% photothermal conversion efficiency (η) at 1 Sun (1000(W/m2)) irradiation, has high potential as a working fluid for DASCs. The study examined how different solar radiation intensities affect photothermal conversion, showing η values of 82.64, 66.73, and 51.11% for the rGO/ N-CQD60/40 nanofluid at 1, 2, and 3 sun, respectively

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