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• In-situ growth of superlattice structure Cu 2 S nanoparticles on MoS 2 nanosheets has been achieved. • l -cysteineconjugated with sulfur defects on exfoliated MoS 2 nanosheets was exhaustively studied. • The intense interfacial effects are formed between Cu 2 S and MoS 2 for 2-Cu 2 S-MoS 2 sample. • 2-Cu 2 S-MoS 2 shows enhanced photoreduction efficiency of Cr(VI) and superior stability. • The strategy is extended to successfully construct CdS-MoS 2 and ZnS-MoS 2 heterojunctions. For obtaining ultra-fine Cu 2 S nanoparticles with reduced recombination of their photogenerated electron/hole pairs, in-situ growth of Cu 2 S nanoparticles on exfoliated MoS 2 nanosheets was achieved in this study. We have exhaustively studied l -cysteine in-situ conjugation with sulfur defects on the surface of MoS 2 nanosheets, which makes particle-level superlattice structured Cu 2 S nanoparticles (average 6.2 nm) well disperse on MoS 2 nanosheets. Further, XPS and Raman analysis confirm that the intense interfacial effects are formed between Cu 2 S and MoS 2 for the 2-Cu 2 S-MoS 2 composite, which significantly promote separation of photogenerated electron-hole pairs and prolong their lifetime, as shown by PL and TRPL. As a result, 2-Cu 2 S-MoS 2 exhibits the fastest photoreduction rate of Cr(VI) (0.0058 min−1) under visible light irradiations, which are almost 8.3 times and 2.9 times higher than that of MoS 2 nanosheets (0.0007 min−1) and pure Cu 2 S (0.002 min−1), respectively. Meanwhile, 2-Cu 2 S-MoS 2 also possesses an optimal composite proportion between Cu 2 S and MoS 2 , which presents outstanding photocatalytic activity among series of x-Cu 2 S-MoS 2 composites. The photoreduction mechanism of Cr(VI) is investigated in detail. As a matter of fact, l -cysteinecoupled with sulfur defects on MoS 2 nanosheets could produce the coordination inner heterojunction interface of 2-Cu 2 S-MoS 2 samples, which greatly improves migration efficiency of photoinduced carriers and ultimately endows it outstanding photoreduction activities. Furthermore, the strategy was extended to fabricate CdS-MoS 2 and ZnS-MoS 2 heterojunctions, implying that this strategy is of promising potentials for designing more active metal sulfides heterojunction photocatalysts. [ABSTRACT FROM AUTHOR]