Defect-driven band-gap modulation in co/Pd co-doped ZnO ceramics synthesized via solid-state reaction route

This work investigates defect-driven structural, electronic, and optical modifications in Zn 1-x-y Co x Pd y O ceramics (x = 0.00, 0.03; y = 0.00, 0.01, 0.03, 0.05) synthesized via solid-state reaction. X-ray diffraction confirmed a dominant wurtzite ZnO phase with minor PdO, Co 3 O 4 , and metallic Pd secondary phases. Lattice parameters increased from a = 3.2282 Å and c = 5.1720 Å (undoped ZnO) to a = 3.2436 Å and c = 5.1952 Å (highest Pd content), while crystallite sizes ranged from 94 to 175 nm. Raman spectroscopy revealed mode broadening and red shifts, indicating enhanced structural disorder. Micrographic analysis revealed a reduction in grain size and an increase in interparticle necking due to co-doping. Photoluminescence measurements showed complete suppression of UV near-band-edge emission and a marked increase in zinc vacancy concentration, from 56.6 % in undoped ZnO to 83.1 % in co-doped samples, accompanied by a decrease in oxygen vacancies from 33.1 % to 10.3 %. Diffuse reflectance analysis demonstrated bandgap narrowing from 3.201 eV to ∼2.80 eV, promoting visible-light absorption. EPR at 77 K identified two Co 2+ related resonances (g ≈ 4.4 and g ≈ 2.3), with the latter decreasing with Pd content. These results demonstrate that Co–Pd co-doping produces a defect-rich ZnO lattice with tailored bandgap and defect chemistry, making these materials promising candidates for visible-light-driven photocatalysis, UV–visible photodetectors, and gas sensing applications. • Co/Pd co-doping via solid-state reaction generates defect-rich ZnO ceramics. • Lattice parameter a expands from 3.2282 Å to 3.2436 Å with increasing Pd content. • Zinc vacancy concentration increases from 56.6 % to 83.1 % in co-doped samples. • Optical bandgap narrows by ∼0.40 eV, enhancing visible-light absorption. • EPR reveals Dysonian (g ≈ 4.4) and Lorentzian (g ≈ 2.3) Co 2+ signals modulated by Pd content.