Ultrathin ZnIn₂S₄ nanosheets with active (110) facet exposure and efficient charge separation for cocatalyst free photocatalytic hydrogen evolution

Due to the relative insufficient intrinsic active sites for hydrogen (H₂) evolution reaction (HER) and fast recombination of photoexcited charge carriers, activity of semiconductor photocatalysts without cocatalyst is still far less than the expectations. Here, we for the first time theoretically and experimentally prove that ultrathin hexagonal structural ZnIn₂S₄ (ZIS) nanosheet is a promising cocatalystfree photocatalyst. According to density functional theory (DFT) calculations, the S atom on (110) facet of hexagonal ZIS coordinating with both Zn and In atoms shows an ideal adsorption free energy of H atom (ΔG_H⁰) of -0.16 eV, which can act as active site for HER. Thus, we fabricate ultrathin ZIS nanosheets (2.6–5.0 nm) under the assistance of trisodium citrate with more (110) facets exposed to provide more active sites. Simultaneously photoexcited electron dynamics studied by femtosecond transient absorption reveals a longer average decay lifetime (359 ps) in such ZIS nanosheets, which indicates an enhanced charge separation efficiency. As a result, HER of the ultrathin ZIS layers in the absence of cocatalyst increases up to 1.94 mmol g⁻¹ h^-1 under visible light irradiation (λ > 420 nm) with AQE at 420 nm of 10.1 %, which is 11.2 times larger than that of pristine ZIS. This work brings a deep insight into structural-activity relationship study and takes a significant step toward the development of cocatalyst free ultrathin 2D photocatalysts for H₂ evolution.