Nanostructured β-Bi₂O₃/Cu₂O Heterojunctions Synthesized via Mechanical Milling for Visible-Light Region Photocatalysis

Visible-light-active photocatalytic heterojunctions based on nanostructured β-Bi2O3/Cu2O semiconductors were successfully coupled by high-energy ball milling, exhibiting enhanced charge carrier separation, transfer, and reaction properties. The optical, structural, and photoelectrochemical properties of the catalysts and their heterojunctions were analyzed to evaluate the coupling efficiency. The optimal stoichiometric ratio of (1 – x)β-Bi2O3/xCu2O (x = 25, 50, and 75 wt %) and the milling time significantly influenced the optical properties, including photoluminescence (PL). A notable decrease in PL intensity was observed for the heterojunction with x = 25%, compared to that of the single semiconductors. This effect was further enhanced with increasing milling time, which is attributed to the mechanical energy input. The formation of nanosized mixed-phase transition heterojunctions was confirmed by scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), and energy-dispersive spectroscopy (EDS). Structural characterization revealed increased lattice parameters and reduced crystallite size due to mechanical milling. Milling-induced structural deformations led to decreased PL intensity and band gap energy. Photocatalytic efficiency was the highest for heterojunctions prepared with shorter milling times; longer milling times introduced structural defects that reduced photocatalytic performance. Flat band potential (EFB) analysis identified a p–n junction with staggered band edge alignment. The heterojunction, which was milled for 60 min, exhibited an absorption range extending up to 1.84 eV. Notably, the 75β-Bi2O3/25Cu2O sample milled for 5 min achieved 98% photocatalytic efficiency under visible light irradiation. Mechanical milling is a simple, inexpensive, one-step process for fabricating nano-sized heterojunctions with well-mixed interfaces that enhance charge carrier separation. While structural defects are detrimental in excess, they can enhance photocatalytic activity at optimal concentrations.