Photo-degradation mechanism of MB dye using sub-10 CdS/RGO nanocomposites under visible (sun) light irradiation. Display omitted •Sub-10 sized CdS NPs evenly embedded on RGO matrix by thermal decomposition method.•Single source (CdDTC2) as precursor source for sub-10 CdS NPs.•The sub-10 CdS/RGOnanocomposites showed excellent photocatalytic activity against standard textile dye solutions.•CdS/RGO nanocomposites showed good reusability after 4 cycles. Quantum dots (QDs) sized sub-10 nanometer (nm) cadmium sulfide (CdS) nanoparticles (NPs) embedded graphene sheets photocatalyst expected significant interest owing to their tunable band gap and high surface to volume ratio. Here demonstrate a single source precursor cadmium diethyldithiocarbamate (CdDTC2) used in presence of reduced graphene oxide (RGO) as template to synthesis sub-10 nm CdS NPs embedded graphene sheet by thermal decomposition method under optimized experimental conditions. The resulting CdS sub-10 NPs modified RGO was characterized with various analytical methods including XRD, HRTEM, UV, FTIR, Raman, XPS, SEM and EDX with mapping. The incorporation of sub-10 CdS NPs uniformly loaded on both sides of RGO was observed by HRTEM micrographs. The photocatalytic performance of the CdS/RGO was studied using a standard textile dye methylene blue (MB) under direct sun light exposure. The results proved that CdS/RGO catalyst exhibit an enhanced degradation rate than the pure CdS NPs. Electrochemical impedance spectroscopic method was employed to examine the prepared CdS/RGO nanocomposites. Results illustrate to CdS species on the surface of RGO can efficiently hasten the separation and postponed the recombination rate in transfer of photo-excited charge carriers. Also, sub-10 CdS/RGO nanocomposites have high surface area and the present method can be used for the preparation of CdS/RGO in large scale.