Plasma-Engineered Hydroxyl Defects in NiO: A DFT-Supported-Spectroscopic Analysis of Oxygen-Hole States and Implications for Water Oxidation

Controlling lattice-oxygen reactivity in earth-abundant OER catalysts requires precise tuning of defect chemistry in the oxide lattice. Here, we combine DFT + U calculations with plasma-assisted synthesis to show how O₂ and H₂O in the discharge govern vacancy formation, electronic structure, and catalytic predisposition in NiO thin films. Oxygen-rich plasmas generate isolated and clustered Ni vacancies that stabilize oxygen-ligand-hole states and produce shallow O 2p-Ni 3d hybrid levels, enhancing Ni-O covalency. In contrast, introducing H₂O during growth drives local hydroxylation that compensates vacancy-induced Ni³⁺ centers, restoring Ni²⁺-like coordination, suppressing deep divacancy-derived in-gap states, and introducing shallow Ni-O-H-derived valence-band tails. EXAFS confirms that hydroxylation perturbs only the local environment while preserving the medium-range NiO lattice, and Ni L-edge spectroscopy shows a persistent but redistributed ligand-hole population. These complementary vacancy- and hydroxylation-driven pathways provide a plasma-controlled route to predefine electronic defect landscapes in NiO and to tune its activation toward OER-relevant NiOOH formation.