Relationship between lattice strain, defect redistribution, and photocatalytic response in Cr3+/Al3+ co-doped ZnO micro–nanostructured systems

The controlled modification of semiconductor oxides via aliovalent co-doping has emerged as a powerful strategy for tailoring structural and electronic behavior in functional nanomaterials. This study correlates the structural strain and defect redistribution induced by aliovalent Cr–Al co-doping in ZnO with the resulting photocatalytic kinetics, focusing on the Zn 1-x-y Cr x Al y O compositions with (x, y) values of (0.00, 0.00), (0.03, 0.01), and (0.03, 0.03). X-ray diffraction analysis, along with Rietveld refinement, confirmed that the wurtzite structure was preserved. The results showed a systematic reduction in crystallite size, decreasing from 103.0 nm in the undoped sample to 71.0 nm and 56.2 nm in the co-doped materials. Additionally, there was an increase in microstrain, rising from 0.122% to as high as 0.316%. Raman spectroscopy revealed progressive shifts in phonon frequencies, indicating lattice distortion. Ultraviolet-visible diffuse reflectance spectroscopy showed a narrowing of the band gap from 3.157(1) eV to 3.130(1) eV. Additionally, Photoluminescence deconvolution revealed a redistribution of optically active defects, with the V Zn -related emission contribution (integrated area fraction) increasing from 41% (undoped ZnO) to 79% (Zn 0.96 Cr 0.03 Al 0.01 O). The Zn 0.96 Cr 0.03 Al 0.01 O sample exhibited mesoporosity with a surface area of 9.4 m 2 g −1 and achieved 89% discoloration of Reactive Black 5 under ultraviolet irradiation in 120 min, outperforming the higher Al-loaded material (85%). Kinetic analysis followed a pseudo-first-order model, with rate constants of 0.0163 min −1 and 0.0117 min −1 , respectively. Reactive species quenching identified valence-band holes and hydroxyl radicals as the dominant oxidative agents. Reusability tests demonstrated a gradual decrease in discoloration efficiency from 89% to 71% and 48% after three consecutive cycles, while X-ray diffraction confirmed the structural stability of the wurtzite lattice throughout the recycling process. These results clarify how aliovalent co-doping governs the interplay between lattice distortion, defect chemistry, and charge carrier behavior in ZnO, providing a well-defined structure–property correlation that is essential for rational design of high-performance semiconductor oxide materials.