Cl-Modified N-Type Cu2O Thin Films for Enhanced Photocatalytic Degradation of Norfloxacin Under Visible-Light Irradiation

Cl-modified n-type Cu2O thin films were prepared on indium tin oxide substrates through potentiostatic electrodeposition and investigated for the photocatalytic removal of norfloxacin (NOR). Cyclic voltammetry and X-ray diffraction identified −0.65 V and pH 5.7 as suitable deposition conditions. Cl addition regulated the nucleation and growth of Cu2O, producing pronounced changes in crystal orientation, surface morphology, and electronic properties. The thin film obtained with 10 mmol L−1 KCl (Cl10-Cu2O) exhibited the strongest (111) preferred orientation and a flower-like morphology. X-ray photoelectron spectroscopy confirmed the presence of Cl species and revealed predominantly Cu+, together with defect-associated oxygen environments and a small proportion of surface Cu2+. The Mott–Schottky analysis demonstrated a transition from p-type conductivity in pristine Cu2O to n-type behavior after Cl incorporation. The optical band gap increased from 1.94 to 2.04 eV, indicating modification of the electronic structure. Among the investigated samples, Cl10-Cu2O delivered the highest photocurrent density (0.101 mA cm−2), the largest open-circuit photovoltage (11.981 mV), and the lowest interfacial charge-transfer resistance. Its apparent pseudo-first-order rate constant reached 0.00539 min−1, approximately 1.61 times that of pristine Cu2O. Scavenger experiments indicated that photogenerated holes were the predominant oxidative species, while ⋅O2− and ⋅OH also participated in NOR transformation. The improved photocatalytic activity was attributed to the combined effects of conductivity-type conversion, controlled defect formation, enhanced charge separation and transfer, favorable crystal orientation, and hierarchical surface morphology.