Structural and optical properties of ZnO and Cu-doped ZnO thin films and the influence of surface defects on glucose oxidation

Zinc oxide (ZnO) and 4 % copper-doped ZnO (Cu–ZnO) thin films were synthesized by the co-precipitation method, deposited via spin-coating, and annealed at 450 °C for 6 h. Structural and surface analyses using X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), and energy-dispersive X-ray spectroscopy (EDX) confirmed enhanced crystallinity and grain growth, with crystallite size increasing from ∼17 to ∼31 nm and surface roughness from 2.8 to 8.3 nm after Cu doping and annealing. Photoluminescence (PL) spectra revealed suppressed oxygen-vacancy–related emission, consistent with reduced defect levels and improved crystallinity. Glucose oxidation, investigated by attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, showed that oxygen vacancies dominate in unannealed ZnO, whereas surface copper oxide (CuO) species drive oxidation in annealed Cu–ZnO through Cu²⁺/Cu³⁺ redox reactions. These results demonstrate that defect engineering combined with low-Cu-content doping in thin films effectively enhances glucose oxidation, offering a simple yet effective strategy for ZnO-based non-enzymatic glucose sensors.