Orchestrating Single Ru Atoms and Second‐Shell Cation Vacancies in CdS Interfaced With Ti ₃ C ₂ T _x MXene for Boosting Photocatalytic Hydrogen Evolution

ABSTRACT Precisely engineering the coordination shells of single‐atom catalysts (SACs) through defect control represents a powerful yet largely underexplored strategy to tailor their microenvironment and activity. Herein, Ru single atoms (SAs) and cadmium vacancies (V Cd ) are synergistically introduced into CdS nanoparticles coupled with Ti 3 C 2 T x MXene, forming Ru‐Cd 1‐x S/Ti 3 C 2 T x Schottky junctions with asymmetric Ru‒S 3 ‒V Cd motifs at both the surface and interface. Notably, the second‐shell V Cd cooperates with the Ru SAs to downshift the p‐band center ( ε p ) and optimize H 1s‐p antibonding orbital occupancy, enabling adjacent S sites to attain a near‐ideal hydrogen adsorption free energy (ΔG H* = −0.03 eV) and a ΔG 2H* value of 0.23 eV for the dihydrogen intermediate. Concurrently, V Cd ‐mediated Ru─O covalent bonds act as atomic bridges at the heterointerface, amplifying the built‐in electric field (BIEF) by 2.56 times and accelerating interfacial photoexcited charge transfer in 1.3 ps. With an ultralow Ru loading of 0.1 wt.%, the optimal Ru 0.1 ‐Cd 1‐x S/Ti 3 C 2 T x ‐1.5 wt.% composite achieves a photocatalytic H 2 evolution rate of 48.58 mmol g −1 h −1 , surpassing pristine CdS by a factor of 45.4, along with an apparent quantum efficiency (AQE) of 16.2% at 420 nm. This work establishes a new strategy for atomic‐level microenvironment engineering of SACs across both surfaces and heterointerfaces.