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 cocatalyst-free photocatalyst. According to density functional theory (DFT) calculations, the S atom on (110) facet of hexagonal structure ZnIn₂S₄ (ZIS) coordinating with both Zn and In atoms shows an ideal adsorption free energy of H atom ($\Delta G_H^0$) 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 AQY 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 hydrogen evolution.

由于氢（H ₂）放出反应（HER）的固有活性中心相对不足，并且光激发的电荷载流子快速重组，因此没有助催化剂的半导体光催化剂的活性仍然远远低于预期。在这里，我们首次在理论上和实验上证明了超薄六方结构ZnIn ₂ S ₄（ZIS）纳米片是一种有前途的无助催化剂光催化剂。根据密度泛函理论（DFT）计算，六方结构ZnIn ₂ S ₄（ZIS）的（110）小平面上与Zn和In原子配合的S原子显示出理想的H原子吸附自由能（$\Delta G_H^0$）-0.16 eV，可以用作HER的活性位点。因此，我们在柠檬酸三钠的帮助下制备了超薄的ZIS纳米片（2.6-5.0 nm），并暴露了更多的（110）刻面以提供更多的活性位点。同时通过飞秒瞬态吸收研究的光激发电子动力学揭示了这种ZIS纳米片材的平均衰变寿命更长（359 ps），这表明电荷分离效率得到了提高。结果，在没有助催化剂的情况下，超薄ZIS层的HER增加至1.94 mmol g ^-1 h ^-1在可见光（λ> 420 nm）下，在420 nm处的AQY为10.1％，是原始ZIS的11.2倍。这项工作为结构-活性关系研究提供了深刻的见识，并朝着无助催化剂的超薄二维光催化剂的发展迈出了重要的一步。