Phase transformation of electrospun CuO fibers by reducing treatment and their photocatalytic performance.

Electrospun copper oxide fibers with tunable phase composition were fabricated by employing ascorbic acid as a green reducing agent. This method enables the progressive transformation of calcined CuO fibers into CuO/Cu 2 O heterostructures and Cu 2 O by controlling only the treatment time, without compromising the original fibrous architecture. Structural, chemical, and optical analyses demonstrate that the reduction process modifies the copper oxidation state and the electronic structure of the material, leading to systematic changes in bandgap values and interfacial characteristics. The photocatalytic performance of the fibers was evaluated using methylene blue as a dye model contaminant under simulated solar irradiation, employing an immobilized photocatalyst configuration. Among the systems studied, the CuO/Cu 2 O heterojunction exhibited superior activity compared to both individual phases, highlighting the key role of interfacial charge-transfer processes. Besides, scavenger experiments were conducted using •OH, •O 2 ⁻, and h + quenchers, confirming the participation of multiple reactive oxygen species in the photocatalytic process. The incorporation of a low concentration of hydrogen peroxide further intensified dye degradation, evidencing a synergistic effect and reaching an efficiency of 94.1%. Kinetic analysis supports a mechanism dominated by more efficient charge separation and enhanced generation of reactive oxygen species at the CuO/Cu 2 O interface. Furthermore, the stability of the CuO/Cu 2 O + H 2 O 2 system was evaluated by SEM analysis after three consecutive reusability cycles, revealing preservation of the fibrous structure. This study demonstrates that controlled phase engineering combined with green synthesis routes constitutes a promising pathway for the design of copper oxide-based photocatalysts applicable to water treatment under simulated solar irradiation.