Integrated Experimental and Theoretical Insights into Defect‐Induced Functionalities of Cobalt Doped NiO for Multifunctional Solar‐Driven Applications

Cobalt‐doped nickel oxide (CNO) nanostructures are synthesized using an eco‐friendly Phoenix dactylifera leaf extract route for sustainable, multifunctional solar applications. Systematic Co doping (5–20 wt%) modulates structural, electronic, and optical properties, enhancing photocatalytic dye degradation, photothermal conversion, and hydrogen evolution. Rietveld‐refined X‐ray diffraction confirms phase purity with lattice expansion ( a = 4.1763–4.1859 Å), increased crystallite size, and reduced microstrain. Fourier‐transform infrared and Raman spectra reveal blueshifted M‐O vibrations, indicating oxygen sublattice distortion and defect‐induced stiffening. X‐ray photoelectron spectroscopy (XPS) and photoluminescence analyses corroborate the formation of Ni 3+ /Ni 2+ , Co 3+ +/Co 2+ mixed‐valence redox pairs, and suppressed radiative recombination, indicating effective tuning of surface chemistry and charge‐carrier dynamics. Optical studies demonstrate bandgap narrowing from 3.14 eV (undoped) to 2.58 eV (20 wt% Co), enhancing visible‐light absorption. Density functional theory calculations validate these trends, predicting a reduced bandgap and preferential Co 2+ substitution at Ni 2+ sites, with energetically favorable defect complexes (V Ni + O i ) promoting charge separation. 15 wt% Co‐doped sample (15CNO) exhibits optimal performance: ≈95% dye degradation, ≈52 °C photothermal heating, and ≈1600 µmol·h −1 ·g −1 . This synergistic enhancement is attributed to bandgap tuning, lattice strain, and defect‐assisted transport, validating a novel defect‐engineering strategy for NiO‐based materials in sustainable applications.