Tunable structure–property engineering and visible-light photocatalytic performance of ZnO:TiO2 nanocomposites synthesized via a solid-state route

Abstract

This study investigates the synthesis and characterization of (1 − x)ZnO:(x)TiO₂ nanocomposites (x = 0.01 to 0.15) using the solid-state method. The structural properties are analyzed using X-ray diffraction (XRD). The XRD analysis reveals changes in phase composition, indicating TiO₂-dependent structural modifications. At low content (x ≤ 0.05), Ti⁴⁺ ions dissolve substitutionally in the ZnO hexagonal structure. Beyond this threshold, the system reaches Ti saturation in ZnO and instead forms discrete Zn₂TiO₄ crystallites as a secondary phase. The crystallite size, estimated via the Debye–Scherrer model, decreases significantly from 50.8 nm to 44.5 nm with increasing TiO₂ content, and reaches 22.2 nm for 3% TiO₂ addition. Fourier transform infrared (FTIR) analysis likely confirms the Ti-doping of ZnO through the presence of Zn–O and Ti–O bonding. The band-gap energy was calculated using Tauc's model based on optical Reflectance data in the UV-Vis wavelength range. The direct band-gap transition exhibits a red-shift from 3.224 to 3.185 eV, while the indirect band-gap transition shows a more noticeable red-shift from 3.121 to 3.021 eV. The red-shift suggests band-gap narrowing, possibly due to defect states introduced by the Ti⁴⁺ cation. Furthermore, photocatalytic tests reveal that Ti-doping improves the methylene blue (MB) degradation efficiency under visible light. Notably, the ZnTi10% and ZnTi15% samples exhibit the highest degradation rates, reaching 87% with an apparent rate constant K_app = 1.19 × 10⁻² min⁻¹, while pure ZnO and TiO₂ remain more active under visible light. This can be related to defects or interfacial states induced by Ti-doping, which act as electron–hole recombination centers.