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 Ni³⁺/Ni²⁺, Co³⁺+/Co²⁺ 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 Co²⁺ substitution at Ni²⁺ 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.