Shallow Trap States Induced by Zinc Vacancies in Wurtzite ZnO for Enhanced Charge Separation and Photocatalytic Hydrogen and Oxygen Evolution Reactions

Hydrothermally synthesized ZnO photocatalysts with controlled cation deficiency were prepared to evaluate the role of Zn‐vacancy engineering in solar water splitting. Pristine ZnO (Zn) and Zn‐deficient ZnO were systematically characterized by structural, spectroscopic, and positron‐annihilation techniques. Zn1 preserved the wurtzite phase but exhibited vacancy‐induced lattice relaxation (100 d‐spacing: 0.285–0.286 nm; 002 d‐spacing: 0.266–0.268 nm) and crystallite refinement from 25.98 to 19.55 nm. Optical studies revealed bandgap narrowing from 3.14 to 2.97 eV and enhanced tail states (Urbach energy: 0.167–0.563 eV), indicative of improved visible‐light absorption. STEM–EDX confirmed Zn deficiency (Zn at.%: 43.06–33.90), accompanied by surface hydroxylation and electronic restructuring of the work function: 4.28–4.02 eV, while valence‐band maximum (VBM)/conductionband minima (CBM) positions remained favourable for H + reduction. Charge‐carrier analysis showed suppressed near‐band‐edge recombination and extended lifetimes. Positron annihilation spectroscopy further evidenced increased vacancy trapping (τ 2 : 362–329 ps; I 2 : 70 to 74%). Zn1 demonstrates superior and nearly stoichiometric H 2 and O 2 evolution under sacrificial‐agent‐free UV irradiation (2 hr), with stable recyclability, confirming intrinsic enhancement of ZnO water splitting via Zn‐vacancy engineering. Controlled Zn vacancies introduce shallow acceptor states that induce valence‐band tailing and sub‐bandgap absorption, acting as transient traps to prolong carrier lifetime and suppress bulk recombination without creating dominant deep nonradiative centers. Simultaneously, Zn vacancies lower the work function and optimize surface adsorption energetics, facilitating charge extraction and interfacial transfer. Thus, an optimized Zn‐vacancy concentration ensures balanced light absorption, carrier dynamics, and surface kinetics in ZnO.