The Influence of Nd³⁺ Ions on the Optical, Photocatalytic Properties, and Surface Morphology of Thin Mesoporous ZnO Layers

Thin mesoporous ZnO and Nd-doped ZnO (ZnO/Nd_X) films were fabricated by a single-step sol–gel and spin-coating process and comprehensively characterized in terms of their optical properties, surface morphology, and photocatalytic activity. Spectroscopic ellipsometry combined with Lorentz–Lorenz analysis and scanning electron microscope imaging confirmed the formation of mesoporous architectures, with a moderate Nd³⁺ content (3 mol%) simultaneously increasing film thickness and inducing a more open, granular surface, whereas higher dopant loading promoted local densification and phase segregation. All films exhibited high transparency (~90%) in the visible range and a slight redshift of the absorption edge upon Nd doping, accompanied by a modest decrease in the direct bandgap from 3.289 eV (ZnO) to 3.268 eV (ZnO/Nd_6). Photocatalytic tests using methylene blue under 365 nm UV irradiation revealed that the 108.0 nm-thick mesoporous ZnO/Nd_3 film achieved 99% dye degradation within 120 min, significantly outperforming undoped ZnO (83%), which is attributed to the synergy between the enlarged accessible surface area and Nd-induced defect states that enhance charge separation, while excessive Nd³⁺ contents generate recombination centers and reduce activity.