Development of co-catalyst free, core/shell structured photocatalyst with ultra-thin shell is of great importance towards the stable and continuous hydrogen (H-2) production, where the shell prevents photo-corrosion of the core for longer stability with continuous H-2 generation. Accordingly, herein, we report a one-step, surfactant free hydrothermal process for synthesis of high-efficient CdS/ZnS core/shell structured catalyst for H-2 evolution under natural solar light. The structural and morphological characterizations using XRD and TEM techniques revealed the formation of phase pure CdS/ZnS system, with core and shell thickness of 395 and 15 nm, respectively. XPS studies revealed that the constituted elements in system exist in their native oxidation states, which indicated the stable structural integrity of the individual phase in the core/shell structure. The synergistic optical properties of CdS/ZnS showed the absorption edge around 500 nm and the decreased PL intensity indicated the improved charge recombination resistance in the system. The parametric studies such as synthesis time, core diameters and shell thickness optimization were conducted to study the formation kinetics of the core/shell structure and their photocatalytic efficiencies. Accordingly, the optimized core/shell catalyst showed around 763 and 2.4 folds superior activity when compared to the pristine CdS and ZnS, respectively. Further, the catalyst showed excellent stability for over 100 h with quantum efficiency of 8.78% under the irradiation of 20 W LED light at 420 nm. Based on the obtained results, the observed improved photocatalytic quantum efficiency could be ascribed to their synergistic effects of CdS and ZnS towards increased charge separation and spatial distributions of the carriers due to their core/shell configuration of the materials. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.