Abstract: Photocatalytic H₂ evolution in two-dimensional covalent organic frameworks (COFs) is often limited by sluggish interlayer charge transfer. Ni single atoms were anchored in the interlayer galleries of a β-ketoenamine-linked TpPa COF by wet impregnation and NaBH₄ reduction. X-ray absorption fine structure (XAFS) and theoretical X-ray absorption near edge structure (XANES) analyses show that the interlayer Ni atoms adopt a six-coordinate Ni-(N/O)₆ environment, whereas the intralayer-anchored Ni@TpPa control has a four-coordinate Ni-(N/O)₄ environment. Spectroscopic and computational analyses indicate that the interlayer Ni sites promote directional charge transfer, prolong carrier lifetimes, strengthen interlayer electronic coupling, and provide a near-optimal hydrogen adsorption free energy. With comparable Ni loadings (0.72 wt% vs 0.73 wt%), Ni/TpPa achieves a hydrogen evolution rate of 9.02 mmol g^-1 h^-1, about 26 times that of Ni@TpPa, and remains stable over four cycles. This interlayer confinement strategy offers a route to efficient COF-based single-atom photocatalysts.

Key words: Interlayer confinement, Ni single atoms, Covalent organic frameworks, Photocatalytic hydrogen evolution, Ni-(N/O)₆ coordination