Structural, optical band gap, and antibacterial properties of sol–gel-derived Mg0.94Ni0.06O nanoparticles

Mg0.94Ni0.06O nanoparticles (NPs) were synthesized by an acid-assisted sol–gel route and evaluated with an integrated structure–optics–microbiology workflow. Powder X-ray diffraction (Cu Kα) revealed a single rock-salt periclase phase indexed by the (111), (200), (220), (311) and (222) reflections, with no secondary phases within the instrument’s detection limit. Lattice metrics obtained from multi-peak d-spacings yielded a=4.212±0.002 Å, consistent with substitutional Ni2+ on Mg2+ sites without symmetry lowering (Fm3ˉm). Line-broadening analysis indicated nanocrystallite sizes of 26–43 nm (mean D≈32 nm), microstrain ε∼2×10−3, and dislocation densities on the order of 10−3 nm−2, evidencing a moderately strained solid solution formed at 550 °C. UV–Vis–NIR measurements showed a strong UV response with a visible/near-IR tail. A Tauc analysis using the direct-allowed form (αhν)2 vs. hν gave an apparent optical onset Eg=3.85 eV (λg≈322 nm); this edge is attributed to defect/dopant states (oxygen vacancies and Ni-derived levels) rather than the intrinsic far-UV MgO gap. Antibacterial activity against Escherichia coli was assessed by agar diffusion (ADM) and spread-plate (SPM). ADM produced measurable halos (representative diameter D=9 mm; replicate mean 11.67±2.52 mm), confirming susceptibility despite nanoparticle diffusion limits in agar. In SPM, treated plates exhibited zero or near-zero colonies (0, 3, 1 CFU), indicating no detectable growth within assay detection limits under the tested conditions. Collectively, the data show that low-level Ni doping preserves the MgO lattice while tuning defect chemistry that red-shifts the optical onset and enables strong, contact-dominated antibacterial performance—supporting the material’s potential in antimicrobial coatings and filtration media.